Hi @TonyH2, thanks a lot for your post here in the community.
As @BenMcr has suggested, you can delete registered devices via the Sky Sports app which you've done, but when you sign in on TV GO it should say something along the lines of the fact you've reached your device limits, but it should offer you the option to replace one with the one you're trying sign in with.
I hope this helps and please let me know if you need further support!
Many thanks
Tom_W