I recently had massive connectivity issues with my ISP and could not connect via PPPoE, despite having correct credentials on the router's internet connection page. 

Further investigation by them revealed - to my horror - that the router was connecting (successfully!) to my previous internet service provider.  It also had the unexpected side effect where my speeds were restricted as I had a slower fibre contract speed with my previous supplier.  Their systems should have kept me off their network, but also my TP-Link router should have equally not been providing old login details from somewhere deep inside it.

So, despite paying my new provider for over 10 months, the router had in fact been connecting to my previous broadband company AND speed profile!!  Anyone else had this?

A factory reset finally solved the issue.

Does anyone know why or where the router kept my old login credentials?  Seems quite a major bug to me!