Hello everyone! I wonder if working with pppoe MTU in 1500 and cause some kind of problem, after a week of problems with some websites that do not show images, our supplier link pointed as a solution to replace the pppoe server MTU to 1500 is that this value is correct ?
grateful for your attention!

Normally PPPoE supports maximum MTU of 1492. I you need more then BOTH server and client need to support RFC4638.

My PPPoE MTU is 1432 but I had to use MSS clamping to reduce MSS on TCP SYN packets to 1386.

if running pppoe server to a wlan Interface - mtu could be 1500.

If run to a Ethernet Interface, With 1500 bytes mtu - it should be set to 1492 due to 8 byte pppoe.

Its possible to set mtu to 1508 troug Whole net and run 1500 to pppoe server. I suppose you have a change mss setting to Your server running at 1500 to have it to work. this makes fragmentation and will reduce speed to Your net.

If I understand your problem correctly, see screenshots in below file.
You need one rule to cover in interface and another rule to cover out interface (same interface on both rules - your PPP interface).

http://www.oram-net.net/public/ROS-MSS.pdf

This is a "hack" to solve the mtu issue. Fragment packages, and ""fill" net With a lot of small packages. Works well on small net, but will make troble on large net. pps will doble, and CPU/ram etc on routerboard will raise a lot - Better to have the correct mtu to Interface if possible