Initially I had some problems, however, you shouldn’t do things when tired. Eventually I worked out where I had went wrong and got things to work after all.
First I had to set the following in Winbox Terminal;
/port firmware set directory=firmware ignore-directip-modem=yes
After I reboot the router, my device showed up as being connected to USB1. Which is what is needed for the next step.
Mikrotik should configure a PPP interface automatically, as such you will need to tweak it to match what your needs are. Those settings below are what I am using for my Telstra dongle in question. I modified the name of my interface to suit my environment.
name="ppp-out2" max-mtu=1500 max-mru=1500 mrru=disabled port=usb1 data-channel=3 info-channel=2 apn="telstra.internet" pin="" user="" password="" profile=default phone="" dial-command="ATDT" modem-init="" null-modem=no dial-on-demand=yes add-default-route=yes default-route-distance=0 use-peer-dns=yes keepalive-timeout=30 allow=pap,chap,mschap1,mschap2
Finally step was to setup a NAT rule to use the new out interface.
/ip firewall nat add action=masquerade chain=srcnat comment="masquerade for ppp-out2 (dongle)" out-interface=ppp-out2
Once this was all done, it worked fine. Very impressed.
Unit pictured below configured and running with Telstra 4G dongle plugged in.