CC can't connect from some networks. Other apps/browser connect work. Please help.

[TheRealJohnAdams]

I have Calibre running in a docker container on my NAS. I use SWAG (nginx proxy plus some extras) to access Calibre from my domain name. I should be able to access Calibre anywhere with an internet connection, and I mostly can:

• From any network with a connection, I can go to calibre.mydomain.me/content-server/, and it works as expected.
• From any network with a connection, I can use Calibre Sync to connect to my Calibre server, and it works as expected.
• From some networks (including my home network, Starbucks wifi, my parents' home wifi, etc.), I can use Calibre Companion to connect to my Calibre server, and it works as expected.
• From other networks (including my phone hotspot and my aunt's home wifi), I get the "Cannot find Content Server" error when I try to connect.

I am not sure where to begin troubleshooting this. The fact that it happens on some networks but not others suggests that it is a network configuration issue. But the fact that it happens with Calibre Companion and not with other apps suggests that it is a problem with Calibre Companion. Any suggestions?

Note: I realize that Calibre Companion has been abandoned. I downloaded the last working version of the APK and signed it to prevent updates. If this bug could be resolved, the existing version of Calibre Companion would meet all of my needs.