Stack Exchange
Separate connections over 1 network cable
Consider the following network setup
                         ┌───────────────┐                                
┌────────────┐           │               │               ┌───────────────┐
│            │           │  2 Port RJ45  │               │               │
│ ISP / DHCP │──────────▶│  Socket Roof  │──────────────▶│ Unifi AP Roof │
│            │           │               │        B      │               │
└────────────┘     A     └───────────────┘               └───────────────┘
                                │  ▲                                      
                                │  │ B                                    
                                │  │                                      
                                │  │                                      
                             CAT6 Split                                   
                                into                                      
                              2x100MBit                                   
                                │  │                                      
                                │  │                                      
                              A │  │                                      
                                │  │                                      
                                ▼  │                                      
                         ┌───────────────┐                                
                         │  2 Port RJ45  │                                
                         │ Socket Ground │     B                          
                      ┌──│     Floor     │◀───────────────────────┐       
                      │  │               │                        │       
                      │  └───────────────┘                        │       
                      │                                           │       
                      │                                           │       
                   A  │                                           │       
                      │                                           │       
                      │      ┌────────┐               ┌────────┐  │       
                      │      │ Unifi  │        B      │ Unifi  │  │       
                      └─────▶│  USG   │──────────────▶│  PoE   │──┘       
                       WAN/1 │        │ LAN/1         │ Switch │          
                             └────────┘               └────────┘                  

Now the issue is there is only one cable run from the roof to the network rack on the ground floor. To get 2 connections out of this I have split the CAT6 cable up into 2x100 MBit using 4 wires each.
So the network is as follows:

The ISP comes in on the roof via a standard RJ45 (radio relay)
The cable is then connected to a 2 port socket (port A) I wired up to split the cable into
2x100MBit
On the ground floor the cable from A is connected to the USGs WAN/1 port.
From the USG LAN/1 port it goes to a PoE switch that connects the rest of the local lan
One PoE port from that switch goes back to the 2 Port socket on port B
Port B of the socket on the roof is then connected to the AP

Now I can't say I really like this setup very much. The reasons this is set up this way is

I am a network noob and probably don't know better
I wanted to power the roof AP via POE without an additional injector (not that important)
I wanted to isolate the ISPs DHCP by splitting it into 2 physical connections and using the USGs DHCP instead (important)

So the question is:
Can I achieve those goals without splitting the cable up into 2x100MBit, e.g. using a USW Flex Mini switch?

The issue with your proposed or current solution is that you lose a lot of bandwidth by splitting the cable, going from 1000 Mbps to 2 x 100 Mbps.
The obvious solution would be to work with VLANs. It would allow you to achieve most of your goals.
Note that I can, due to a lack of experience with PoE, not speak to the PoE part of your requirements, however you mentioned that PoE is not that important. I cannot say if my proposed solution would allow PoE or rule it out, however.
Working with VLANs will allow you to:

Work with just one cable between roof and ground floor.
Not lose any bandwidth
Keep your LANs A and B completely separated.

What you need for that is a device that is capable of 802.1q based VLANs. Some examples of devices, that usually are VLAN capable, are:

Managed switches
'Smart' switches
Some WiFI Routers

You will need two of those devices. One will be on the roof and one in the ground floor. I will call them SwitchRoof and SwitchGroundFloor
This is what the configuration will look like:
SwitchRoof

Port 1: connects to ISP modem
Port 2: connects to roof AP
Port 3: connects to ground floor (all pairs, 1000 Mbps)

SwitchGroundFLoor

Port 1: connects to USG
Port 2: connects to ground floor switch*
Port 3: connects to roof (all pairs, 1000 Mbps)


Note, depending of your physical layout and setup, this device (SwitchGroundFLoor) and the ground floor switch can be the same device. Likely, the Unifi  switch in the ground floor is 802.1q VLAN capable and you would not need to get an additional device for the ground floor.

How to configure the switches (both on the ground floor and on the roof)
VLANs

VLAN ID 1 will be your LAN (B)
VLAN ID 2 will be your ISP/modem (A)

Port configuration

Port 1 will be untagged in VLAN2
Port 2 will be untagged in VLAN1
Port 3 will be a trunk port with VLAN1 tagged and VLAN2 tagged

How does this work?
The connection between roof and ground floor will be called a trunk. If data belongs to VLAN1 (B) it will be tagged as such and sent over the trunk and the receiving switch will remove the tag and send it out to port 1. Likewise, if data for VLAN2 (A) is sent over the trunk, it will be tagged as such and the receiving switch will forward it to port 3.

Thanks! I thought about VLANs before and read into it, however I always wondered how that would work with only one switch on the roof. This trunk setup makes a lot more sense!
