Can you tell us what modem is in use? As well, where is the mumble server located, in the U.S. or Canada. I'm wondering if you're running into issues with servers somewhere down in the U.S. causing problems. There is one thing you could check and that is what side of the electrical panel both wireline devices are on, right or left. If they don't sit on the same side, see if you could move one of them to a location where it will be on the same side. Then run whatever length of ethernet cable you need to make that work, either into the first device or from the second device. That probably isn't how it can be left permanently, but just for test purposes see if that makes any difference. One other thing you can check is the cable signal. Here's a link to my CGN3 DOCSIS WAN data showing the downstream and upstream power levels and signal to noise ratios.
Can you log into your modem, navigate to the DOCSIS WAN page, copy those table and paste them into this thread, just for a quick check that will hopefully rule out any cable issues. Copy the tables only, and none of the other data that sits above the downstream table such as the WAN IP Address and others.