Android Verizon LG G4 on straight talk? (Only H signal?) + How to root?

[Aether Lion]

I have a vs986 LG G4. These phones apparently come compatible with all GSM sims. However, I don't get 4G signal at all using Straight Talk (AT&T). Also, service in general for it is a lot more sparse and uncommon than it is with the same sim inserted into my unlocked G2. Any idea?
There is also an issue with not being able to use the mobile hotspot since it isn't a Verizon sim.

Also, what is the easiest way to root it? I downgraded it to Lollipop.
Thanks!