Scaling in a small geographic area like Plymouth is always tough. You're likely hitting a wall because the audience pool on Meta is just too small. The algorithm runs out of new people to show ads to very quickly, so your frequency shoots up and costs go through the roof.
I'd say you've probably maxed out Meta for that location. The best way to scale further is almost always to expand to other ad platforms to tap into a different audience.
We've run a fair few app growth campaigns and have seen this happen before. For one client, we managed to scale their app to over 45,000 signups by adding Apple Search Ads and Google Ads into the mix. Apple Ads in particular can be really effective for installs as you're catching people right when they're looking for apps in the app store. It's a different kind of intent.
If you're dead set on making Meta work, you could try opening up your targeting to be much broader within Plymouth. Let the algorithm have more room to work. But honestly, I'd put my money on testing new channels first. The audience just might not be on Meta in the numbers you need.
Hope that helps!
