Unable to connect to shared drives after setting up Airport Extreme Bridge and Catalina Update.

I have an old Macbook Pro in my bedroom that has been connected to my home network thru an Ethernet cable that snakes around the apartment. (Wifi seems to break bad on it in a way I could never solve, so just using ethernet wound up being easier.)

That cable became severed (in an unrepairable way) and rather than replacing it, I decided to try to set up a bridge using an Airport Extreme I recently acquired.

This worked the way I wanted, with the Airport Extreme setting it up easily.

However, since then, I've had some file sharing issues.

If I connect thru Ethernet, I do not see any of the computers on my network:

If I connect with the WiFi, other devices on the network show up:

I can then connect to them. IF I turn off WiFi after connecting to my iMac, I don't lose connection to the shared iMac folders.

(there are some windows computers not showing up here as well, but for now, I'm tackling just the mac part).

HOWEVER, that's not the only problem....

I can connect to the iMac's startup drive and internal storage drive (two seperate drives), but When I try to connect to any connected USB drives, I get an error "The operation can't be completed because the original item >drive name< can't be found.", despite the USB drives being listed as being there:

To complicate matters further, the iMac was just upgraded to Catalina from Mojave. So I don't know if this USB drive issue is a problem with the AirPort Extreeme OR Catalina OR bother.

Looking at both the iMac and the Airport Extreeme, I can't find any settings to adjust. or change.

Can anyone suggest what the problem may be, or how to fix the issues?