Lots of old posts about
this, that I recall in the distant past. Zinc gluconate works for me - zinc lozenges in general seemed to be popular if I remember correctly - Bonjela mouth ulcer jelly - and there was another one I can't remember, I think it had lignocaine in it which of course was good pain relief.
There is a theory that says mouth ulcers are a sign on vitamin B deficiency - which one I am not sure, probably B1 or B2 - and that it is brought on by having lots of processed sugar in the diet, which uses up the body's whole store of this vitamin.
I certainly find that my mouth ulcers suddenly come whenever I start eating lots of sugar-laced foods, and go just as rapidly when I stop eating those foods.
But of course what is true for me will not be true for everyone !