Please allow me to provide some some context. Republic is able to provision its newer phones for cellular coverage with one of two network partners. One partner operates a GSM network and is T-Mobile. Republic’s other partner operates a CDMA network and is Sprint. In and around your immediate area either partner provides good coverage. In and around Tioga, CDMA coverage (Sprint network) is considerably better than GSM (T-Mobile network).
Here are my suggestions for you. Your current SIM is GSM and will indeed move to the new Moto X4 presuming it’s compatible (it almost certainly is). Once past initial setup of the new X4 and with your current SIM installed, you’ll need to download or update, then open Republic’s app. Doing so, will present an abbreviated activation sequence that will move service from old to new Moto X4.
Once the new Moto X4 is up and running using your current GSM SIM, if you wish to improve your coverage when visiting Tioga (without hurting coverage at home), please see here:
The CDMA SIM would provide the same coverage for your Moto X4 that your father has on his older Republic phone.