Thanks for your suggestion. I didn't realize SIP Enable was on the NAT Forwarding -> ALG menu page

I have a Gigaset C470 voip phone on my network which has stopped working. Is the Archer VR400 blocking SIP on port 5060 & RTP on port 5100 by default? I cant see where I can check what network ports...

ok, thanks for clarifying this. Could I add a load balancing router? Something like the TL-R470T with both modems connected to the load balancer?

Dear forum, I have a TP-Link Archer VR400 V3 connected to the Internet via the DSL line setup for ADSL2+. I also have a 3G/4G LTE modem (LG2120) which I use as a backup when the ADSL line fails but I...