Welcome to Android Central! Find out from Safelink which towers are used in your area. You need to make sure the phone supports the network bands used by those towers' carrier (i.e., Verizon, Sprint, US Cellular, AT&T, or T-Mobile).
You can find the phone's band support here:
https://www.gsm-specs.com/specifications/umidigi-f1/. Since it doesn't appear to have a CDMA radio, it can't be used on Verizon, Sprint, or US Cellular -- so when you talk to Safelink, as if they use AT&T or T-Mobile towers in your area.