How to protect a diamond ring so the stones don't fall out when it takes a knock
Covering a cut stone with something like clear nail polish will diminish the shine of the stones. And you have to expect the polish to break or flake of at some point.

Instead you should go to a professional jeweller and explain the problem to them. They can have a look at the tiny metal protrusions that hold the stones in place and reshape some of them if needed. Or they can glue the stones in place (on the backside) with a professional glue that doesn't break off.