Failover on 24.7 + USB Tethering

[asw2012]

I've got a strange problem here on my 24.7 box.
I have setup a failover to my normal cable internet WAN with a 5G line.
It's working but.....

when I plug the USB tether into the OPNsense box, the phone / data drops down to ~3-5Mbps D/L speeds and ~40% loss + 1000ms ping.
when I plug the USB tether into a single computer, speeds are 200+ Mbps D/L (normal speeds).

I test each directly from the phone, using OOKLA.

What's with the OPNsense box and the slow speeds?