GSMLocationNlpBackend

https://f-droid.org/packages/com.google.android.gms backend that uses OpenCellID.

Location calculation is done onboard and works without network connectivity. The
cell-tower based lookup for your current location is implemented via a included
binary cell database extracted from OpenCellID.

This "NetworkLocationProvider" works without network connectivity and will never
post your data anywhere. You are thus encouraged to help OpenCellID to gather
more cells in order to improve this project.

https://f-droid.org/packages/de.enaikoon.android.keypadmapper3 can be used to gather cell information for
OpenCellID.

Warning: This app drains battery and seems no longer maintained.