I have the RM-1096 which is the 'dual sim lte' model. After further testing, I've come to the conclusion that both my wireless carriers just have awful service in my area (AT&T and T-Tmobile). I pulled both sims and tested in my 520, the speeds were significantly lower. I was able to get around 20+ Mbps on my 640, but on the 520, in the same area, it capped around 6 or 7.
The reason I get 4 or 5 on my 640 at work is because I work in an area with a lot of interference I guess. My 640 identifies the network (using the app 'Net Check') as LTE on my 640, but not the on the 520.
At the end of the day, I could care less about the speeds, I just want to make sure I'm able to get as much connectivity as possible, since I'm often in remote areas.
I'm mildly surprised I can't exceed 20 Mbps with either carrier, as I'm located in Southern California in a major metropolitan area, but it is what it is.