In PPPoE scenarios, if the length of an IP packet carrying the DF field is greater than the MTU configured on a device, the device discards the packet, which compromises services. To resolve the problem, adjust the MSS configured on the device.
Set the TCP MSS on the firewall to be smaller than 1452.
[USG] firewall tcp-mss 1400
When packet lengths are limited, you can adjust the TCP MSS to resolve the problem of slow network access.