The are able to join the server, but they can't do anything. They just sit there frozen in place.
I installed a Forge 36.2.30 1.16.5 on an old gaming laptop with Ubuntu 20.04 as a server. I had the server on an older piece of hardware and wanted to upgrade it to a newer laptop. I made sure minecraft was let in the Ubuntu firewall, all the mods matched, I even checked if the port forward was working properly (it is). I can join just fine on the local network, but they can't play after joining for whatever reason.
The only thing that changed is what hardware the server was on. The last one worked without an issue.
Server Specs:
DELL G7 7588
CPU: Intel i7 8th gen 8750H
GPU: Nvidia 1060 w/ Max-Q Design (6GB)
RAM: 16GB (alloted half)
SSD: 128GB
OS: Ubuntu 20.04.4