In addition to Nick's comments...

Are both of the 4G connections using sim-cards going to the same provider? If so, are they on the same sub-net?

Be good to see an extract of /var/log/syswatch just before, during and after a connection goes down. (within code-tags please)

What is the reason for using fail-over rather than the default Multi-WAN weighting with both WANs active at the same time?