Some implementations use GRE. It’s safe to allow:
/ip pool add name=l2tp-pool ranges=192.168.100.2-192.168.100.254 mikrotik l2tp server setup full
If you want clients to access the internet through the router (full tunnel), add masquerade rule: Some implementations use GRE
Ensure this range does not overlap with your existing DHCP server pool. mikrotik l2tp server setup full