Surf Soho MK3 not routing traffic over USB tether to Netgear Nighthawk m1

I’m on firmware 8.0.2, and I cannot get my surf soho to route traffic over USB.

I can get the soho to the point it says it’s connected, and I can talk through the soho to the web admin on the Nighthawk, but it wont route traffic.

If I connect the soho wifi as wan to the M1, it will traceroute and work mostly fine:
traceroute 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets
1 airstream (10.3.0.1) 1.234 ms 0.957 ms 0.923 ms
2 attwifimanager.routerlogin.net (172.20.250.1) 3.966 ms 3.794 ms 2.577 ms
3 172.26.96.161 (172.26.96.161) 171.017 ms 150.627 ms 42.155 ms

If I connect over USB, things get interesting:
traceroute 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets
1 airstream (10.3.0.1) 1.300 ms 0.961 ms 0.931 ms
2 * * *
3 * * *
4 * * *
5 * * *
6 * * *
(Note, it doesn’t even hop to the M1 on 172.20.250.1)

However to add even more strangeness, I can traceroute (and even use HTTP) to trace to the M1:
traceroute 172.20.250.1
traceroute to 172.20.250.1 (172.20.250.1), 64 hops max, 52 byte packets
1 airstream (10.3.0.1) 1.269 ms 0.917 ms 0.948 ms
2 attwifimanager.routerlogin.net (172.20.250.1) 3.467 ms 2.069 ms 2.711 ms

To me, this looks like the connection for the USB didn’t actually place the default gateway into the soho router.

When I ssh into the router, I can see a bit more:

get wan

WAN Connection [2]
Connection Name : USB
Connection Status : Connected
Connection Type : Modem
Connection Method : PPP
IP Address : 172.20.250.4
Default Gateway : 172.20.250.1
DNS Servers : 172.20.250.1
MTU : 1428

Also, the router knows about the M1 in the arp table:

support arp
? (172.20.250.1) at cc:40:… [ether] on WAN7

The device doesn’t have any static routes configured or too much else interesting going on.

Why isn’t the USB modem getting into the route?

Same results on 8.1.0