I have come to understand that configuration for the SIM are done on the modem and not on the SIM injector.
And the instructions I have found are very scant on how to set this up, with the basic resources that I have found (and to summarize them on a single page.) to be…
This PDF (How to setup HD2 Dome with local SIM Injector)
which only leads up to connecting the SIM injector to the Dome (but does not outline on how to set up the SIMS)
And this youtube video MAX HD2 Dome with SIM Injector | Quick start guide
Where you can find how to set up the SIMS
And also I didn’t find the Sim Injector in the peplink firmware download page so luckily in the forum (thanks @SteveTaylor) I found this link for the latest Sim Injector firmware.
And if you look in the Remote SIM Management page (of the modem) you will see that
Beyond putting APN credentials there is no further options, (in my case urgently important) Data Roaming!
loaded now in the HD2 Dome is firmware 8.0.2 build 4407
and the injector has firmware 1.1.55