Ola Mson77 veja o que diz o manual do mikrotik relativo ao service-ports.

Service Ports

Submenu level: /ip firewall service-port

Some network protocols require direct two-sided connection between endpoints. This is not always possible, as network address translation is widely used to connect clients to the network. This submenu allows to configure Connection Tracking 'helpers' for above mentioned protocols. These 'helpers' are used to provide correct NAT traversal.
Property Description

name - protocol name ports (read-only: integer) - port range that is used by the protocol Example

Suppose we want to disable h323service port:
[admin@test_1] ip firewall service-port> set h323 disabled=yes
[admin@test_1] ip firewall service-port> print
Flags: X - disabled
0 ftp 21
1 pptp
2 gre
3 X h323
4 mms
5 irc 6667
6 quake3

[admin@test_1] ip firewall service-port>

O fato que tanto FTP quanto GRE/PPTP se desabilitado no service ports pode impossibilitar a comunicação mesmo que nao seja uma comunicacao MK->PC ...