First, PMTUd does require ICMP to be forwarded to the machine that sent the traffic. But if you allow RELATED,ESTABLISHED the required ICMP will be let through. And NAT with connection tracking will get it back to the right machine.

You're likely looking for this magic iptables line, though:

iptables -I FORWARD -p tcp -o ppp0 -j TCPMSS --set-mss 1452 # MTU-40 for IPv4
# use the correct outbound interface in place of ppp0

Likely, your router does that automatically.

First, PMTUd does require ICMP to be forwarded to the machine that sent the traffic. But if you allow RELATED,ESTABLISHED the required ICMP will be let through. And NAT with connection tracking will get it back to the right machine.

You're likely looking for this magic iptables line, though:

iptables -t mangle -I FORWARD -p tcp -o ppp0 -j TCPMSS --set-mss 1452 # MTU-40 for IPv4
# use the correct outbound interface in place of ppp0

Likely, your router does that automatically.