I'm using the readytoconnect.fs username and the password must work as the dsl is connected...
No, the DSL connection to exchange doesn't use your creds; it's all done by electricity. You creds are only used at the next stage of authenticating you onto the EE network & then onto the wider Net.
Also your router's MAC is not kept anywhere, but its serial no. is recorded so that so that it may initially auto configure the router to your own creds when the BB Username is defaulted to 'readytoconnect@fs'.
However, as any router may be used on EE BB all you need to do is set the BB Username/pwd to your own creds so it won't need to auto configure and it will then connect.
I'm not clear if you are on just ADSL with just a BB1 or on Fibre with a BT fibre modem + BB1? Does it still connect with your old BB1?
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC