365 Business Email Domain - Outlook sign in issues.

I've recently signed up to Microsoft 365 business and after learning (quite frustratingly) that although the sales literature states "free custome business email" (or something along those lines) - I had to purchase my domain seperately via a 3rd party, in my case GoDaddy as they are reccommended. Anyway, once I understood this, purchased a domain and followed the complex (for a newbie like me anyway) instructions on how to change things from my ".onmicrosoft" address to my new custom domain, I thought I was in business. However... Everything appears fine in the admin centre and when in my apps such as teams, excel and word everything now shows as my custom emai address. The problem is when I try to log into outlook it just doesn't want to work. I get a recurring log in pop up asking for the password. Does anyone know where I'm going wrong?

PS - I' not sure wether the two are linked but although my teams shows the correct custom domain email address, I can't seem to change from being offline to online ?

Thanks in advance.