I am running a Bionic32+25 to experiment with @wiak 's add-on WDL sfs and I discovered while setting up the upupbb that the eth0 and eth1 ethernet devices are not detected after repeated attempts, and hence not configuring or connecting.
Swapped out the kernel to a 5.6.4-rt3 (i686) I had compiled some time ago, then rebooted and the machine connected to the network automatically.
Machine is a DELL PowerEdge R210 II and this is the first time Bionic32 has done that. I tried all 3 network tools several times over but the cards were just not detected. No worries it's going again but I thought I'd mention it.