HTC 10 Battery replaced and still having issues

Hi everyone. I made the stupid mistake of purchasing an HTC 10 a while back. Initially it seemed to be just fine but after a few months I realized the battery wasn't lasting very long and looked into it. The app Accubattery said my battery's health was 30%. Anyhow so I got the battery replaced and it doesn't seem to have helped if anything it may have made it worse. Accubattery says the new battery health is 21% at the moment. The repair place said the reason this is happening is likely due to the fact the main board was apparently water damaged (they said something internal indicated water damage had happened at some point even though it was all working now). They gave me a full refund and said we could try replacing the battery again but it might not help.

Any ideas what else I can do to try to bring this phone back to life? Is the whole water damage messing with the ability of the battery to charge/function properly legit?