Pppoe packet sniffer config, possible?

I have an interesting configuration challenge at my friend's in-laws house.
The long story:
They had a DSL service for years with the phone company but then this company had financial issues so the service degraded , failed but was neve repaired. In the rush my friend (really not a technical person) choose a fiber provider for them . He was absent when the installation was done and he was happy that the wifi network had the name and password he asked for over the phone but did not realise that he needed the admin password of the main router! So far I have not been able to recover this password because if I reset it, it will loose the pppoe crendentials, an info I haven't been able to gather from the ISP web site or help line! How can I be sure about pppoe? I tried a temporary router directly connected th fiber "modem" box connected to the main router and I did not work with DCHP client configuration.
my goal:
I am thinking of inserting an openwrt flashed router acting as switch in between the fiber "modem" and the main router but with the ablity to sniff packets with a computer connected to this sniffer router. Possible? Would I be able to get the credentials?