I closed down my business as a computer tech in 2007 when I was forced out by serious injury, so I'm only a hobbyist these days and many things have changed since the days when I had to 'think'.
However, I noticed that on my main machine Warpinator is using the address related to this:
wgpia0:
While the LAN traffic is using the ip address related to:
en01 (wired ethernet)
While on the other Mint system, Warpinator is using the IP address related to:
wlp2s0: (same as eth0 on main system)
and the PI is using:
wlan0: (same as eth0 on main system)
Which makes Warpinator the Odd Man Out in the network.
So now if I can work out what the hell wgpia0: is, and why, Warpinator is using the IP address associated with that, instead of the IP address of the computer (that this instance of Warpinator is installed on), I might be able to getit going.
Unfortunately I only now worked this out, and I have some reading to do.
I cannot see anywhere in Warpinator to change the IP address it tries to use, to the address everything else on the LAN uses, so I'm guessing there is a config file somewhere
Time to hit DuckDuckGo again..
EDIT:
That was quick. It looks like wgpia0 might be related to my VPN.
Warpinator might be getting confused by some setting in the VPN App.
I'll try Warpinatror with the VPN turned OFF and see what happens..
EDIT:
If I am connected to the VPN, Warpinator shows the IP address of the current VPN server - and will NOT show other computers on the LAN.
If I disconnect the VPN, Warpinator reverts to the Local IP address that all the other computers see - and still will NOT show other computers on the LAN.
So the VPN is not the culprit !!
And port 42000 is allowed for tcp and udp, so the Firewall should not be the culprit, assuming Warpinator uses tcp or udp. I wonder what it uses?
And the LAN traffic is working perfectly, so the LAN is apparently not the culprit . . .
U P D A T E:
I finally got MY Warpinator solved. Unfortunately it's not going to help the OP . .
on BOTH Mint 20.1 computers.
Now all the computers on the Network can see each other, VNC Remote View from the Main computer to the Servers on the others, and share files between each other..
VNC worked ok with just Linux Networking, but I had to fall back on Samba to get sharing. At least now it all works.