I've had my RS988 LG G5 for a couple weeks, and like it much better than the Nexus 5X it is replacing, except for this LTE reception problem. I'm seeing the same symptoms described here, using a Cricket SIM card.
In my office, where every phone we've used in the past 3 years has received an LTE signal, my new G5 shows HSPA. Outside, it
usually switches to LTE, but not always.
To analyze, I put a different Cricket SIM into my old Nexus 5X, and did some checks using the Network Cell Info app. First, sitting in my office, the 5X (top screenshot) always shows LTE on Band 17, and the G5 (bottom) always show HSPA on Band 2.
Later, I went out to run some errands and took both phones. I observed the 5X never wavered from LTE, using bands 2, 4, 5 and 17, depending on my location. The G5 stayed on LTE 90% of the time during the trip, but switched only to bands 2, 4 and 5.
This convinces me that for some reason, the RS988 running Nougat 7.0 is unable to use Band 17, and therefore has limited LTE reception on AT&T / Cricket / other AT&T MVNO's.
Some people say that this was not a problem before the Nougat upgrade, but I can't vouch for that, having never used my G5 on Marshmallow.