MR1100 shows fast speeds but actual downloads are still slow

Looking for any help or suggestions here: I just got an MR1100 as a backup or alternative for our awful home internet speeds. I've got it up and running on ATT and on speed tests, it's showing a nice 100mbps download. But when I actually try to download files - video I need for work, the downloads are really slow - slower than my home wifi is. Any suggestions on why that might be happening or adjustments I need to make to get the actual download speeds to match speed tests? Thanks in advance!