The question is also what bitrate can guarantee you your ISP in the agreement? It's generally very different than theoretical bandwidth.
I presume DSL/LTE router need, like a mobile phone, to receive well LTE network. Maybe you have a way to check that.
New product are almost everyone IPv6 compliant but IPv4 is still use everywhere. I think IPv6 isn't for tomorrow.
What is the output of your phone ? Usually phones connects to the router/box with RJ11 cable.