I developed those painful miserable little mouthsores twice. The compound my onc prescribed did not help much. Then I read this on a different site. I haven't had any sores since. It's worth the effort.
1 tsp salt (I use Himalayan, but any is good)
1 tsp baking soda
Mix in warm water.
Swish in mouth 15 seconds and spit out. Repeat at least 4 times. Then I swish with plain water to get the salty taste out. I do this now 3 times a day. Haven't developed any sores since. 🙏