Will ext4 formatted disk configured as Time Machine over SMB be recognised as Time Machine disk when directly attached? I've bought a new router (LinkSys WRT32x), flashed OpenWRT onto it, and set it up as a NAS server with a disk attached (formatted in ext4 because OpenWRT is a Linux distro).
I have the disk recognised as a Time Machine backup disk through Samba 4, and I'm now backing my MacBook Pro wirelessly.
However, if I have to recover from this partition, as it is formatted in ext4:

Will I have to do so wirelessly through my router (supposing my Mac can still access the LAN in whatever state it's in) or will the disk be recognised as a Time Machine backup when plugged into my Mac?
What will happen if I can't restore wirelessly, will my Mac even recognise the ext4 disk?

Any knowledge or advice appreciated, I am keen to learn.
Yes, you will have to do via the router. It could be wirelessly or via a cabled ethernet connection. This is normally not a problem as macOS Recovery supports Wi-Fi and restoring from network Time Machine backups.
If you plug the disk directly into the Mac it will not be recognized as a Time Machine disk, and you will not be able to access its contents without a third-party ext4 driver.