VPN connection problems with an Xfinity xFi Gateway modem

I'm trying to troubleshoot a recent issue that's popped up for my parents when they're connecting to my OpenVPN server and trying to open files on my server (typically Word and PDF files). They can connect fine as usual, but after browsing the folder structure on the server, Windows explorer stops responding when they go to open up a file (and sometimes they can't even open a folder before it stops responding). The blue circle just spins continually, and the Microsoft Word splash screen never actually opens.

I've narrowed it down to their Comcast gigabit network as the issue occurs wired directly to the modem but everything works properly if they use their phone's hotspot instead. It also occurs on numerous different computers.. Restarting the modem has no effect. The issue comes and goes. It won't work for a while (several hours/days), and then all of a sudden everything opens fine. Usually it's the morning when things don't work properly and by the afternoon they're back to normal.

I advised them to try the two suggestions in this thread, https://forums.xfinity.com/t5/Your-Home-Network/VPN-keeps-on-dropping/td-p/3323693 1. Disabling xFi Advanced security (it already was). and 2. Setting the gateway to Bridge Mode (which didn't work).

Any ideas what the problem could be?

Thanks

OpenVPN GUI: v11.14.0.0

OpenVPN Server: v2.4.8

Comcast Modem: TG3482G (Gigabit speed plan)

Server Connection: Verizon FiOS gigabit