Sounds odd. Forgive me I'm a little rusty as I haven't had DSL for a few years.
My recommendation though would be to take the router out of the loop for now and get everything set with the modem doing the PPPoE.
Then add the router back in set to DHCP but be sure the router has a different IP range - So if the modem is 192.168.1.1 then set the router to 192.168.2.1
You can turn off NAT on the modem. Only one device should do the NAT.--
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