ZBT-WE826-T speeds drop when downloading?

Hi I was wondering if someone can help me.
So we have a ZBT-WE826-T router with a sim card with sprint in it but its going a little weird. For some reason speedtest will say its getting 30mbps, but once you start downloading something it will go to speeds of like 1mbps (Downloads will go to like 40kbps). Once you go on youtube it will load 4k youtube videos just fine as well as 1080p 60fps just fine. I have a friend that has the ZBT WG3526-P router and they are having the same issue. Is there something about these routers that sprint doesn't like? It would be awesome if someone can help me!

Since someone with experience with these devices or with Sprint is bound to ask, are these running OpenWRT (or LEDE)? If so, what version? Do you know what protocol is being used to talk with the modem (serial/AT, QMI, ...)?

LEDE 17.01.4 r3560-79f57e422d
and for the serial/QT I’m not 100% sure but I think it’s DHCP.