I've setup an L2TP tunnel on a Teltonika which works great. This is setup as the default route out.
The only issue I am having is for some reason DNS requests will not go out over the L2TP tunnel but instead over the LTE/Mobile circuit.
The reason I know this to be case is I can see the DNS requests coming into the DNS server and the IP presented is that of the LTE circuit instead of the L2TP tunnel. It seems it's only DNS that the Teltonika is doing this for. All other traffic is going over the L2TP tunnel except for DNS.