Sold Apple Watch - EE cellular unpairing

Hi

i recently sold a Apple Watch series 3 cellular following an upgrade

I had a short trial of the cellular on U.K. EE Option which was cancelled at he ended of trial several years ago.

the new user is unable to connect this to their account as it stated it is locked to previous account/ phone (/the original phone was also sold )

the watch was fully erased and reset prior to sale

can anyone advise how this can be resolved