I’m not sure what I’ve done, but somehow the physical interfaces on my Routerboard RB951G-2HnD are backwards.
Port 5 = Interface 1
Port 1 = Interface 5
Running Routeros 6.25
/interface> print
Flags: D - dynamic, X - disabled, R - running, S - slave
NAME TYPE ACTUAL-MTU L2MTU
0 R ether1-gateway ether 1500 1598
1 RS ether2-master-local ether 1500 1598
2 S ether3-slave-local ether 1500 1598
3 RS ether4-slave-local ether 1500 1598
4 S ether5-slave-local ether 1500 1598
5 RS wlan3 wlan 1500 1600
6 X 58.96.58.18 sstp-out
7 R LAN bridge 1500 1598