It would be much better to answer this question if I had a picture of the x-ray of that tooth. However from the data you've provided...
if the x-ray shows a root canal treatment that is short of the the entire length, you should get it re-treated.
Saving the tooth is important however given that you have had gum swellings that have been resolved with cleaning, implies that you might have an ill-fitting crown(cap) which might have food lodgement.
Get that replaced as well.
Try saving the tooth with a re-RCT.
Good luck.
Next Steps
Get consultation for Re-RCT and to check if the tooth will be restorable adequately.
Health Tips
Regular checkups to keep oral hygiene in check