OAB (over active bladder) Back.

My brother has been suffering from OAB since 2021. He took homeopathic medicine for a pimple that gave him this condition. Most doctors misdiagnosed the issues as they were not aware of it. Between 2022-2024 we consulted many doctors, took many tests all reports were normal. Recently we visited pgi Lucknow , their doctor suggested diatrin 7mg. He completely recovered in one month but took the pain killer for the back issue (zerodol & myotop) then again OAB returned. Now diatrin 7mg not working. We tried other medicine like Mirago 25mg(mirabegron)  which gave him side effects like low pressure and was not much effective. Please guide which medicine will work in this case. How can he be cured completely. Taking diatrin for 1 month. He completely got cured . But taking a pain killer again brought the condition back.