I’m not sure about your request
You have enabled PortForwarding in ISP Router:6881 to Raspberi Pi Port…
But The networks are like:
Case 1
ISP Router port 1 to Mkt port 1
ISP Router port 2 to Rapsberry Port
or Case 2
ISP Router port 1 to Mkt port 1 and Mkt port 2 to Rapsberry Port?
In the secon case you can remove the NAT on mikrotk ana bridge al port or must add the dst nat on mikrotik
Solution Case 1:
ISP Router Port Forwarding to Mikrotik
than
Mikortik IP FIREWALL NAT add DST NAT → dst port 6881 → action → dst nat to raspberry pi IP
Soltuion Case 2:
ISP router Port Forwarding to raspberry pi IP
Mikrotik bridge new bridge → add all port → disable dhcp server on mikrotik