I'm running Ubuntu with latest Bitcoin Core.
For some reason I don't have this problem with Windows v0.18.0, which connects to the 192.168.1.100 node no problem.
Which is it now? Latest is v0.18.1. You should test both versions on both systems. If v0.18.0 works on Ubuntu as well, then a regression must have occurred. Once you do, update the thread and I'll file it. Maybe also wipe your debug.log one more time before you do the tests and upload them here (or pastebin).
Did you create the file called bitcoin-qt.conf? (see post under me)