Hey myetttt,
You have this correct, yep! That would definitely be the thing to try at this point.
Just unlink my Xbox account from the EA account that it's currently on, re-link it to the account that it was on and if that doesn't work, then the Xbox account data has been deleted.
It's worth noting this would never have transferred anything from one (inaccessible) EA account to another (main) EA account, so this would be a matter of seeing if it's still on that older account rather than being able to update things for the newer one sadly.
Certain Apex fan websites used to track your progress or status can give you a general indication of what was on the account, but they're more like a snapshot of the account at a certain point in time rather than a live indicator of what's currently there. The only way to see what's still actually on the account would be logging into Apex on that specific EA account to see what turns up.