Mtech,
Have a look at Understanding and Configuring PepVPN/SpeedFusion as this explains Speedfusion well. as does https://download.peplink.com/partner/SpeedFusion_Best_Practices.pdf
For speedfusion itself to work you must have a peplink device at either end licensed and configured to use speedfusion. See Speedfusion confusion.
The total bandwidth depends on the configuration - take a look at Link aggregation between brances - #2 by TK_Liew .
If there is a large imbalance in the data rates between your DSL and your LTE it can give you a lower overall bandwidth. See Peplink EU Distributor | Frontier BV | Connectiviteitspecialist