It is possible to authenticate a router in the network via pppoe but without releasing the internet access to the devices connected to it.
The router connected via pppoe receives and releases access to the network hotspot for the stations connected to it
its no problem having a mixed dhcp and pppoe at same out interface. (if this was your question) you can have some device that get an ip via dhcp (or have a manual set ip) mixed with pppoe for authentiation. You can also have ip to CPE’s from the same pool of ip’s
my goal is to give registered customers access to gain access through hotspot authentication on any router authenticated via pppoe
this way you could control the routers connected in pppoe and give the customer the freedom to migrate between the routers without having to enter a new password in their device