I just swapped from a Samsung S8 to a Sonim XP8, simply moved my SIM card. The Sonim was purchased new from Verizon.
The Sonim will not go into 3g mode when traveling in rural areas, it seems the phone only likes 4g. I also carry an separate Samsung S8 for my work, it will enter 3g mode in rural areas so I know that network is available.
I did the *#*#4636#*#* and played with several different CDMA network settings, no luck.
Sonim tech support tolde that the phone gets network settings directly from the SIM card, the phone itself can support CDMA.