Importance of the Path MTU Diagnostic Tool for some websites are not working issues
For PPPoE or other unusual MSS / MTU environments, we have a PathMTU Discovery diagnostic tool in the UI (System > Diagnostics) which can be used to diagnose the network environment to set the correct MTU on the WAN interface. This tool is new in SonicOS 184.108.40.206. A simple test, followed by setting the MTU according to the results, may resolve any functional issues the customer is facing. See below the output from me running the test. You can use any public hostname or IP which is pingable for your test, such as google.co.uk .
We have for over a decade auto-adjusted the effective MTU for PPPoE by the required 8 bytes, but sometimes more is needed.