Exact same issue for me too even in 2020...Customer care was useless as they redirected me from one dept to another and I already lost 3-4hrs trying to activate flex and get this XRE-00251 issue resolved. I've decided to give it up finally as it is a free service anyways. it won't be their top priority.
To be specific it's the smaller boxes for bedrooms etc. I'm having this nightmare with. I've spent 3 hours with 3 different service reps in India. None solved the problem. Did NOT have this problem setting up main box (with same error code) when I spoke to the rep in USA , and it only took 23 minutes.