My Moto G from just three years ago also carries the limitation that one SIM gets 3G and the other SIM gets 2G. It's a way to offer two SIM support without actually adding more radios.
The issue came up in another thread recently as well. It looks like at least some of the LTE phones can let the 2nd SIM use the 3G radio, though there may be some limitations there as well. Basically, don't count on any limitations to be clear from the manufacturer's documentation. People have to test it to find out.
Here is the other thread. I'd go for a foreign LG X Power dual SIM these days, for an affordable handset with dual SIM and also great battery capacity.
Honestly, BLU is on the very low end of the spectrum when it comes to phones. As already posted, the brands like ASUS, One plus, ZTE, Huawaei, etc. are very solid phones. Currently using the zenfone 2 on AT&T without any problems. you can look at zenfone 3 deluxe, ZTE axon 7, or even the cheaper ZTE axon 7 mini. These are all very good dual sim devices.
I recently bought a BLU life one x2 and it limits the 2nd sim to 2G. Nowhere is this mentioned. Not in the spec, and not in the manual.
Could people who has a dual sim phone state whether it limits one of the sim to 2G (i.e. won't work with AT&T)?
BLU life one x2: limits one sim to 2G, hence doesn't work with AT&T sim for voice calls.
Here's the reference, and I'm sure I read about this elsewhere as well:
I know the OP mentioned OnePlus, but fwiw, I am reasonably happy with my OnePlus 3. Both SIM slots support LTE. With the latest software release (nougat), it also supports T-mobile wifi calling.
"I didn't get fat by accident. This was a personal choice. " - Kevin Gillespie