@sallyNULL >ISPs do not like to disclose PPPoE credentials
I haven't run into this yet myself, but nowadays I would probably go and crack that router open and see if I can find a UART header to access a command line, or if that fails, dump the NAND with a NAND dumper and then look for the credentials.

I've found that NAT hole punching nowadays barely works for p2p provided other hosts aren't NATed (i.e. at least one other client is binded to a port-forwarded port).

Really, we've needed IPv6 since 30 years ago, but noooo.