Try add this to your router
/ip firewall mangle
add action=change-mss chain=forward new-mss=clamp-to-pmtu passthrough=yes protocol=tcp tcp-flags=syn comment=“Clamp MSS to PMTU for Outgoing packets”
add action=change-mss chain=output new-mss=clamp-to-pmtu passthrough=yes protocol=tcp tcp-flags=syn
OR
/ip firewall mangle
add action=change-mss chain=forward new-mss=1400 passthrough=yes protocol=tcp tcp-flags=syn tcp-mss=1401-65535