In the connection settings where you chose PPPoE, make sure vlan tag is NOT ticked as enabled.
It's greyed out and says "VLAN ID cannot be disabled when Ethernet Connection is enabled"
Hmm, odd. It ought to say VLAN ID cannot be enabled rather than disabled when Ethernet connection is enabled.
The VLAN tag 101 is a function of the modem (ONT in your case)
Yes of course, I've made sure to reboot after every config change.
I finally got it working today. I have no idea why I was receiving the VLAN tag error but rebooting and resetting everything got rid of it. I did the setup over ethernet one more time and it worked. Thank you everyone, got there in the end!
Glad you got it sorted.