I think there may be some confusion here.
I’ve seen nothing to indicate T-Mobile is sunsetting its’ 3G network in October. What I have seen is T-Mobile won’t allow new activations on its’ network unless the phone(s) being activated are certified for VoLTE on T-Mobile’s network beginning in October. To the best of my knowledge, phones already active on T-Mobile’s network aren’t immediately impacted.
I’ve also seen nothing to suggest Sprint’s legacy LTE network (now also owned by T-Mobile as the result of the T-Mobile/Sprint merger) is to be shut down in October of this year. In fact, T-Mobile’s current timeline for shutting down Sprint LTE is June 30, 2022. T-Mobile has indicated it intends to shut down the legacy Sprint CDMA (older than LTE technology) network on or about January 1, 2022.
All of the above said, both of the Moto G7 series phones you reference are 4G LTE and VoLTE capable, so, no, there is no need for an immediate upgrade. I also don’t understand why your phones are showing 3G as a preferred network. Let’s see if we might address that. As a starting point, please advise which Republic SIM type is in each phone. The how to on that from Republic is linked here:
For others reading, it’s true changes are coming to the wireless phone industry generally and Republic specifically. There is, however, no need for immediate concern. Much of what one might see online is speculation and not all of it is particularly informed. I’m confident Republic will tell us what we need to know when we need to know it.