Troubleshoot

MTU MSS

Testing max MTU

The important thing to remember is that an ICMP header is 8 bits so the ping test from CMD should be whatever number is your max without fragmenting PLUS 8 which gives your value for the router.

https://www.sonicwall.com/support/knowledge-base/set-mtu-in-vpn-environment-in-case-of-throughput-issues/170705131319789/