I was using the old Connect Box (600 Mbs) without problems for many years in Bridge Mode, but after updating to the Giga Box (& 1 Gbs) a few days ago I'm having no end of connection problems. Lot's of drop outs, DNS failures, streaming freezing after 10mins, etc. I can connect to websites using a Swisscom 4G Hotspot but the same website will fail to load at all when connected to the UPC network through Wifi, but then at the same time other sites will work (e.g. Google). I've tried different DNS servers (UPC, 22.214.171.124, 126.96.36.199, etc). still no go.
I've cleared routing tables, caches, restarted everything many, many, many times and it still has problems - is there anything on your end that needs to be reset/updated for it to function properly in Modem Mode (/ Bridge Mode)? Nothing else changed on my network (Unifi setup) but since installing the Gigabox it's been no end of problems.
FYI my entire network is IPVv4 using NAT with private addresses internally.