Hello to Everyone!
I would like to make my Mikrotik routers completely invisible from the outside, but I have L2TP VPN service between routers. How can the Mikrotik VPN server recognize that the request is coming from my other Mikrotik if client IP is variable? Can I use mangle dscp as a criteria for the firewall rule or are there better ideas?
Thanks in advance.

You can use firewall filter rules in input chain and enable cloud DDNS by Mikrotik, and store the domain in an address list, then limit the firewall rule to this address list.
If you want to do this way but don't know exactly how to do this, reply here and will help you configure that.