@cre8toruk I'm having a similar issue with 2 different sites with a 5100 and 7100. They're both on the same ISP, and a tech there has indicated there's a firmware glitch with the modems affecting VPN / VoIP traffic... Everthing else works, pings through vpn, vpn doesn't drop, internet slows down a lot - but still up. SMB and Domain Auth seems to be affected the most. Reseting the modem seems to fix the issue for a while, but then it'll stop working for SMB shares usually some random time later.
Have you had any luck finding your issue?