@patrickg
Allow me to disagree, from the OP report, the issue Is something in the Mikrotik acting as a router.
Now this probable (unknown) mis-configuration Is causing some trifling effects and the ISP router firewall Is anyway acting as filter.
If this hypothetical mis-configuration creates also a security risk, setting the ISP router as bridge would remove this current firewall protection.
@msaleiro
Follow the instructions here and post your configuration of the AC3: