In order to establish a SpeedFusion tunnel you need to have a public IP address on at least one of the units. Since the MAX unit is using cellular you will need this on the Balance 310 router. I suggest to put the DSL modem in bridge mode and let the Balance 310 router handle the PPPoE, this way the WAN IP address should be a public one. Your ISP will be able to help you with this as you will need the PPPoE credentials to put in the 310.