Did you clean it when you got it? Not to be an ass, but you say you don't know much about the insides, and you're new to the rifle.
Everyone reads the manual, familiarizes themselves, completely cleans and lubes newly purchased rifles before they fire them, right??
I can't speak for your LMT specifically, but I do know that many rifles and parts are 'preserved' for medium term storage before they leave the factory. This is not ready for firing. All new AR type rifles should get a proper teardown, thorough cleaning with solvents as required, complete bolt disassembly and cleaning, and sometimes a lot of elbow grease for the barrel. It should be assumed that any new firearm should be inspected and prepped as necessary before firing.
Generally, prep new rifle, including good lubing, and shoot it. If stoppages occur, isolate the problem, mags first if reason for the stoppage is not obvious (new/uncleaned/unlubed would be considered be considered obvious

), mags are an easy 1st thing to rule out. Go from there.
All said, you sent your rifle and your mags in and got them back, I'm sure you're good to go now anyways.
Cheers
DT