I would like to help with this, but there isn't enough information here to know what the cause of the problem is. I would say that it's rare, but I can think of some possible causes. like wireless interference, router misconfiguration (for example use of port triggering instead of port forwarding or port mapping), use of a VPN protocol like wireguard that doesn't handle large packet sizes, malware protection that monitors and blocks network activity, etc.
I can help you interactively via IRC.
Otherwise, please post mainlog.html and userreport_hwdetect.txt from each computer. I would suggest that you try connecting both computers with ethernet cables and try connecting to someone else's hosted game, but the problem may not even be at the network at all. Maybe it's caused by competing or disrupting software on one of the computers. I also suggest testing other networked applications, specifically peer-to-peer UDP games like ioquake3.