A new Balance 20x running firmware 8.1.0 build 4938 runs just fine with its WAN port connected to the LAN port of an existing router. However, when the Balance 20x is put directly online, it will not talk to a Netgear CM600 cable modem (Firmware Version V1.01.14).
I tried powering on the router with the modem running and the reverse (powering on the modem with the router running). Both failed. Also tried firmware 8.0.2 with no luck.
Had very similar problem with a Surf SOHO MK3 and the same modem. Surf SOHO runs fine on firmware 7.1.2 but firmware 8 would not talk to the modem. I connected a Chromebook directly to the modem and it worked fine.
So, seems like firmware 8 is the problem. It starts to connect to the modem, but then the DNS Health Check fails. Will open a ticket…