you might need to add a magle rule to adjust the MSS of new tcp connections
/ip firewall mangle
add action=change-mss chain=postrouting comment=“CorrectMSS Size” new-mss=1452 out-interface=all-ppp passthrough=no protocol=tcp tcp-flags=syn
This is assuming your outbound(internet interface) is a PPPoE session otherwise change the out-interface to the correct interface.
Depending on the connection you have you might be able to tweak the 1452 value
Hope this helps
The MSS in that rule should be 40-48 bytes less than the MTU, or you could use the option “clamp MSS to PMTU” to calculate it automatically.
You forgot to include your configuration export so it is impossible to tell you what is wrong with your setup…