You can try the below to isolate the issue :
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You mentioned when the issue happen , SpeedFusion connection is not affected, you can test forward all traffics into the SpeedFusion tunnels instead of directly route via the Cellular WAN , this should able to bypass the issue.
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Try to set the WAN health check using HTTP, this will able to trigger the WAN health check failed and the cellular connection will be reset like 5 minutes when WAN health check is failed. This should able to force the cellular WAN to reconnect (may need your help to confirm this)
The above just a temporary workaround until service provider fix the issue