First, your experiences are one of many reasons why I don't recommend Republic and balance so many recommendations by others here with the downsides of the service. Also for the record, they used to permit three plan changes a month. This is pure anti-customer garbage.
Anyway, have you seen
the guide (
unabridged here) yet?
No matter where you go, the next hurdle you're going to encounter with your formerly beloved carrier is potential porting out issues with your numbers. Either Republic will do something between sitting on the port request so long that it'll cause problems, or depending on the MVNO and parent network, you may encounter porting issues back out to your next provider as your number is no longer considered a mobile number but instead landline number. LNP (local number portability) is supposed to be universal between the three DID (direct inward dial phone number) types here in the States, but you'll find that portability still runs into issues, and it's usually due to who/where you're porting out of. Occasionally, I've heard of some number ports back out of Republic going reasonably smooth, and I hope you're one of them... but don't be surprised if it doesn't. Most of those positive porting out reports are usually of people bailing during the trial period.
Regarding where to go to from here, you mentioned that you used to be with AT&T. This means a couple things: 1) AT&T reception works for you, and 2) it's possible you may still have the handsets. Regarding point #2, if you do, I suspect you didn't get the phones carrier unlocked from AT&T before you burnt that bridge. Hopefully (and thankfully due to carrier unlocking becoming legal in this country again), you can pick up cheap carrier unlock codes through Ebay or something for your old AT&T handsets if you still have them. Just do due diligence on the seller and read the instructions thoroughly if you do. Sadly, Republic's handsets are proprietary, so as you mentioned you don't actually own them, can't unlock them, and can't take them with you elsewhere. Yet another real world downside of this provider, and I'm more reiterating this for other potential Republic customers who might come across this thread than telling you this specifically... but back to the phones and providers.
Why I'm recommending an AT&T MVNO is due to ensuring a happy experience and building on your past experiences. If you go with a T-Mobile or Sprint MVNO, you're going to deal wit a drastically different reception footprint. Now, if you know with some authority that, say, T-Mobile coverage will work for you already, then consider something like Ting; otherwise, stick with what you know. I'd also recommend going GSM with T-Mobile MVNOs, even if you leave AT&T, because then you have a greater handset portability freedom between all GSM networks. You go with a Sprint MVNO, you're going to be limited to taking nearly any phone you get only to other Sprint MVNOs. Verizon's a slightly different bird, but what few Verizon MVNOs there are, they are the most expensive options of the lot. Let's keep focused on the GSM end, and AT&T for compatibility and expectation sake.
That said, of the GSM providers, AT&T MVNOs are overall more expensive than T-Mobile ones (though Ting is one of the more expensive, unless you're doing multi-line and moderate group usage). As such, it's best to know what your actual average monthly usage needs are and find a plan from one of the recommended AT&T MVNO providers that fits those needs for the right price and sufficient overage cushion. You should also learn to embody the following philosophical idea:
PAY FOR WHAT YOU NEED. Most people don't need mobile phone service as much as they think, especially data (but you probably know that already), so pay for what you need. If most of your calls are done at home, don't use mobile minutes...
learn how to adapt on the SMS and data usage ends, and
effectively incorporate VoIP technology on your terms. Combos like Truphone as your mobile provider and Rebtel or Localphone for VoIP services with a good smartphone can be both trivially easy to set up and powerfully cheap to operate with greater flexibility than Republic ever gave you if you don't have an irrational fear of technology.
And that brings us finally to handsets. If you find yourself needing new handsets and keeping in mind the GSM advice... get a GSM handset and buy carrier unlocked with at least GPRS/HSPA 850 and 1900MHz band support. T-Mobile still uses some 1700 and 2100MHz bands around the country, but they're mostly on the same frequencies as AT&T now, especially for high speed data in metro areas. Shop with a mind towards what you need in features, try to buy a handset that you can replace the battery on yourself,
buy used/refurbished if possible, if you go Android again
go for a handset with official CyanogenMod support to extend lifespan further, but don't be afraid (again) to
pay for what you need... just remember,
need does not equal
want.
Any other questions, feel free to ask... but that should give you everything you need to find the best solution on your own instead of having someone blindly tell you what you should do without knowing your actual needs. After all, that's what lead you to Republic in the first place.