TK,
re-reading other posts on the forum like this one, Speedfusion MTU 1600 for SRM VMWare - #3 by Travis , I believe all WANs must have jumbo frame support even if the traffic is encapsulated in Speedfusion.
I can’t use MBX with 4x 1500 MTU cellular connections ina SF tunnel to transport jumbo frames.
@Travis can you confirm this?