Advanced Security and Minecraft

Hello, I have a pretty weird issue that I'd like to get some feedback on before I start buying stuff to try to make a solution. Recently, my partner and I moved apartments and signed up with Xfinity home internet. We have a modded Minecraft LAN world and we'd like to avoid having to try to turn it into a server if we can. For a while it was working fine, and then suddenly every time I tried logging into his LAN world, I would get timed out and he would lose internet entirely for about 5 minutes. After poking around for a bit, we realized that if we turned advanced security off, the problem stopped. This worked for a bit, but now, even with advanced security off, it keeps happening. Every time I go to log in I have to go and turn advanced security on and then off again in order to make it work. 

The modpack is AllTheMods10 since I don't have a good way to copy and paste the entire list. I'm using the rental equipment from Xfinity. Both computers are wired directly to the equipment. Please let me know if any other information is needed.