With the zyxel running as a full router and being responsible for the PPPoE session some commands are required using telnet to get full 1500 byte packets.
The changes to the firmware were primarily to enable 1508 frames in
bridge mode when using a separate router.
I tried last year to automate the process when using them as full routers, but of the two people who tried the modification neither had positive results, you can read about it in this message in the jumbo frames thread:
https://forum.kitz.co.uk/index.php/topic,21273.msg392603.html#msg392603Its been a while since I have tried it, but from the above post the only command required when running the modified firmware should be:
ifconfig ppp2.1 mtu 1500
If this command works, but you would like it to be persistent and dont mind debugging a little we could try out a new firmware with the simple script from that post.