Brubble, We are saying the same thing!
To give an opinion regarding your unknown: shrinking the MTU by 12 bytes gives more header room, thats all. This would also be what is needed if your wireless MTU was set to 1500. A proper, full pppoe packet is 1512 bytes with no mss clamping. If mss clamping to 1480 or smaller is enabled then your pppoe packet should be 1500 or less.
the “cant reach half the internet” calls are usually because the client is trying to exceed the max packet size for the wireless link with the pppoe headers. VPNs usually use smaller packet sizes (typically 1000 bytes from general snooping) and this ensures that the packet doesnt get filtered due to size after the headers are applied and generally ensures no in route fragmentation, though fragmentation is still possible just unlikely.
in M_Al-Mutterdi’s case reducing the pppoe max packet size could help or at least reduce a possibility, however he must also reduce the mss-clamp size to be smaller than the pppoe mtu by at least 8 bytes.
Link MTU should be more than 1500 on an ISP network, epmp can do 1700 and should to ensure no fragmentation on your network. Not sure about your upstream providers but ours have no problems with 1700 for MTU size from us. but that is also something you need to discuss with them too. There is also your network policy to factor in too, ours in this regard is simply large is good but too large is not good. We commonly see packets with MTUs larger than 1536 bytes and we take advantage of the 1700 bytes MTU available with our inter-NOC backups. But that is also our opinion and not always best practice either.