I use Savacol (alcohol free) - you will need to check with the pharmacist because Savacol has some specific instructions about its use. The cancer care nurse suggested using Caphosol. I believe Caphosol is quite expensive, costs about 75 dollars and is not readily available. I did not buy Caphosol. I also apply Manuka honey on the mouth sores and keep my mouth open for a while (quite a funny sight for the family). I also had painful gums, my friend who is a nurse suggested applying coconut oil on that and it did work. Hope this helps.
https://www.sobi.com/en/products/caphosolr#