I have the same problem with my 2-way T1 UK. Actually, it works with one router, but not with many others. The Sonoff POW works fine on all routers, and the computers and phones also work fine on all routers. It is only the T1 UK that doesn't work on the newest routers. I will continue looking for settings that are different in the routers, and post a new post if I find some clues.
I eventually managed to get my Sonoff working by conecting it to a range extender, which I set to broadcast a different SSID
I have the same situation: have a router from Vodafone (Huawei HG8247H) and can get my Sonoff basic to work in another router. Curisouly, I have a sonoff SV and it works