This is currently what I am trying:
Username : bthomehub@btbroadband.com
Password : bt
Connection : PPPoE
VLAN :
I get exactly the same problem as you however if I use my Google nest router, everything works absolutely fine
I can see my OTN Modem keep disconnecting and reconnecting the port so I would imagine as you say the Meraki is using the wrong username and password authentication
I have added this to my ticket I have with Meraki
currently to get this working, I have to have my Google nest Wi-Fi router in Bridge mode connected to my Meraki router then I have access point connected to that…..
I do not like the fact I require three bits of hardware to get my network online whereas if I remove Meraki, I only need one.