Linksys WRT1900ACS with Fiber (Telekom incl. Entertain) - no connection?

I've been using several AVM FritzBoxes with DSL or cable, but after moving I got FTTH by Telekom in Germany. My old hardware is able to deal with it, but only up to 100mbit/s. So I decided to buy a Linksys WRT1900ACS. I want to install LEDE on it, but first I tried to connect with the stock firmware to be sure that it works. Unfortunately connecting fails and I have no hint why. I hope to find same help here and to get the proof that the hardware is working before installing LEDE and preventing myself from the possibility to give the hardware back to the vendor.

I have fiber by Telekom in Germany including EntertainTV. With my old FritzBox 7362SL it was no probling setting up the connecting by configuring one LAN-Port to a WAN-Port, connecting the fiber modem, defining username+password and setting up the maximum up- and downstream speed.
For the 1900ACS I edited the InternetSettings (IPv4), but there is only the option to insert my credentials. As the router has a dedicated internet-port I am using this to connect the fiber modem. Restarting the modem (unplugging) and the router did not help either. Unfortunately I dont get any hints from the logs within the webfrontend. Is there somewhere else to search?

If it's possible to set up the router I may have an additional issue: I am using EntertainTV which means that my TV signal is coming via fiber and is broadcasted with multicast in my network. I have switches which can manage multicast (IGMPv3), so devices like my printer are not flooded by traffic. I could not find the info if the 1900acs router is able to handle IGMPv3, but when trying to use it as an wifi access point only, my smartphone had some issues like streaming music from the internet was possible but mails and even a browser not. After disconnecting the router everything was fine again.

I hope for help, even if it's still on the stock firmware, so I can be sure everything is also possible after installing LEDE. Maybe you also have other suggestions where to ask for help.

Thanks in advance

I guess you should post here the exact details of how you configured it on the FritzBox 7362SL and how are you configuring it on the 1900ACS.

Ah, telekom, as far as I can tell you need to use PPPoE and probably VLAN7 (and for entertain potentially VLAN8 until you are switched to the new BNG access nodes; until now FTTH customers all seem to be stuck on the old BRAS access platform though).

Good luck!