The problem seems to be due to food lodgement between 25 and 26. While placing crown usually that gap should be avoided. Some unavoidable gaps will be there for that you must use floss or interdental brush to avoid food lodgement over that area. So that we can avoid both caries formation and gum problems, bone loss problems etc.
Next Steps
If your doctor has taken xray and advised for rct means you have to do it no other way of solution to get rid of your pain permanently. And one more extra solution if you are affordable you can do this.. after completing rct in 26 u can remove crown from 25 and replace the crown as joined/attached 25 and 26. So that you can avoid food lodgement between 25 and 26 in future and there will be no occurence of reinfection.
Health Tips
Use floss or interdental brushes whenever you have food lodgement between any two teeth.
Always gargle after every meal.