If you do an lspci then you should be able to find out which WIFI card you have and you can search for the driver.

lspci returns nothing at all, I've never seen that before

Me neither! Weird...

The driver is rtl8xxxu.

Thanks, at least I know what to look for now

Hmm, I have the driver but when I try to install it manually it returns with "Unknown symbol".
It would have been compiled from source with the kernel, so I don't know why it won't load.

There is other weird stuff going on with these systems on the NanoPi-M4. I have been trying to activate X11VNC using a separate administrator account instead of the 'pi' user account on the system. For the life of me, I cannot get the startup script for calling up the VNC server executed, although it runs perfectly fine when I run it in the terminal. Adding it to the autostart folder in ~/.config doesn't help either. All the permissions for that folder should be set correctly. The very same script with the very same method executes and connects perfectly fine at startup when I log in and autostart it as user 'pi'.
Happens only on Lubuntu, though, not with the Ubuntu image.

Maybe you are encountering similar permission problems when you are compiling the kernel on those systems.

I don't think so, I built the OS by following the Armbian instructions. That uses a desktop Ubuntu 18.04 image to cross-compile for ARM.
I built it in a VirtualBox VM.

I had nighmares with VNC too, nothing I tried would work properly. I think I tried every single package available with VNC in the name.
In the end I got it got it working fairly easily using tigervnc-standalone-server and a systemd unit file to start the service

Have you considered downgrading the kernel to 4.15?

I know that the cameras work without a hitch with that kernel on a different mini-PC.

I'd love to, but the only images I can find are either stable branch 4.4 or dev branch 5.3 ones.
FriendlyARM haven't released anything later than 4.4 for the NanoPi M4 and that's where Armbian pull the kernel from.

On the dev branch USB works fine, but I can't get WiFi to work. I can mostly live without it, maybe a USB WiFi stick will work for the few occasions that I need it. I'll try that out over the next few days.

I did find (using wayback machine) an old Armbian dev branch based on 4.19, but I haven't had a chance to try that one out yet. Maybe that will be OK, but as the kernel hasn't come from FriendlyARM, I bet there will be features missing

Check out the synaptic package manager after installing Ubuntu MATE on the Nano-Pi. I skimmed over it quickly last night and I though kernels 4.15 and 4.18 were listed there.

I may be wrong, will have to check again tonight. If downgrading were possible, that would solve a lot of problems.

Perhaps installing Ubuntu 16.04 could be the solution?