It sounds like an issue with your proximity sensor being covered or dirty. If you have case, try removing it and test again. At least we can check that off as not being the issue if it does not fix it.
You say it just started…
Was there a (again sorry) case, a screen protector, a new app, or an update that was installed as this issue started for you?
Device help instructions for your Moto can be found here How to Access Motorola’s Device Help App
Here a page that will show you the model# needed for your X4 to work with Republic
Moto X4 Tech Specs
To know if you can switch SIM’s you need to know if you a have a GSM or CDMA in currently.
This will tell you how to find out: How to Tell If a Phone Is Active on GSM or CDMA
If you have a GSM, you should be good to go. If you have a CDMA SIM, you will need to order a GSM to activate the new phone.