The best way to use SpeedFusion is nearly always selectively - so just sending traffic through it that you want to be super reliable, in combination with load balancing for all other traffic.
It is likely though that your situation can be improved with some adjustments. I would suggest you use dynamic weighted bonding since you are using multiple cellular connections (turn it on on the Fusionhub also), I would also advise turning on DSl/Cable Optimization under QoS > Application on the balance which helps when we have asynchronous links.
Also, since you are using a balance I would set the upload and download speeds on the WANs to be representative of best case for those links too.
Those things should help. If they don’t help enough then your cellular connections will need some analysis to work out what’s going on to help us pick the right techniques to mitigate the underlying problems.