would it be easier to have the Ethernet to USB adapter used for a WAN3 connection on a Draytek Vigor 2860...

In my situation I have 3 x WAN connections, 2 of which are VDSL from the same ISP, utilized via the ADSL/VDSL connection and the other VDSL is via a Draytek Vigor130 on the WAN2, My third WAN is a Ubiquiti AirFiber link which I want to use for the QNAP server for Downloading and uploading only ( load balance Policy on the 2860 for QNAP to use wan3 only )

Would this be possible as this Airfiber connection fluctuates between 5Mb to 45MB both up and down ideal for the download server than general Internet , streaming etc