First time posting here. I apologize for having to use English -- my German is not that good (yet!). I hope you can help me regardless

I have a ZTE ZXHN H268N router, which was provided by A1 and is connected to an ONT socket on the wall. I want to replace that router with my own router.
I set my username & password as this page suggests, but both my routers (a Netgear & a Mikrotik) fail to connect.
There is one more option called "Service Name". It is optional, but important for a PPPoE connection to work. Does anyone know if "Service Name" should be left blank or if it has a specific value for A1?
And other than that, do you have any other suggestions to try?
Thank you very much!

Nikolaos