Try running a continuous ping and see if that keeps it up. If not then try a small data stream or bigger packet sizes. That will be the easiest, but next I would rule out the carrier and try a prepaid or data only sim from T-Mobile or AT&T. If that still has issues possibly try a different USB modem.
https://documentation.meraki.com/MX/Cellular/3G_-_4G_Cellular_Failover
There is a list of supported USB modems. Most people are going to tell you to run a cellular device/modem that has e
ethernet handoff that you can plug into a WAN port on the MX.
I have run this modem on T-Mobile on MX64,65, and 67, which are all NA models and is practically plug-n-play
https://www.amazon.com/gp/product/B01M0JY15V/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1