No you don't do port forwarding for an IoT device.
The Sonoff is typical of many of these devices in that they support only the 2.4Ghz band. The hub 3.0 needs separate SSIDs for the 2.4GHz and 5GHz bands, do this and connect to the 2.4GHz band.
Have you managed to get this resolved?
This should be a doddle. Do it on a pc/laptop rather than a phone. When you go into the settings and erase the 2.4 SSID name and change it to anything you want (end it xxxx2) then erase the password and type in any new random one you like and have just created, then repeat it for the 5GHz network (use xxxx5 and the same password you used for the 2), then save/apply changes and away you go.
I have exactly the same issue. New hub as old one broke. Two Sonoff devices paired beautifully with everything before new hub. Now won't work.
I have separated the 2.4 and 5 into two WiFi networks both work fine. I have paired the Sonoff devices as per instructions, both recognise the 2.4 WiFi and pair to the hub but remain stubbornly off line.
Thank you for your post. I'm sorry to hear this.
Would you be able to disable the 5G connection to see if this helps?