Having a read of the manual it is a different start up than the Dometic.............
So you have pressed and held the start button till the lights dim and the switch has a green light ......... then pushed the confirm button, select source and push confirm again then select cooling level then confirm again............ if your not doing the button thing in the right sequence then it probably won't go, on gas they can take a awhile to fire up as a delay can be imposed when going from 12v to gas, so you don't have the fridge fire up while at the garage fuelling.
Manual I sourced :
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiIsYXWxKfxAhWk7nMBHXJCDmkQFjACegQIChAF&url=https://www.caravanrvcamping.com.au/assets/files/700-03880_UserManual.pdf&usg=AOvVaw161z8bTevtbZHA2kxSlPkl
its a pdf download...
12v is only for when hooked up to tug with engine running so 240 or Gas at all other times but unless the fridge is cold it will take 10 or 12 hours before it will get cold, feel around the exhaust or top vent outside after 30 mins or so you should feel some warm air.....
I would attempt to get it started on 240v then switch to gas once its working, just as a test ............... after about an hour from a warm start you should be able to feel the freezer section getting cold................................ also these modern units do require a 12v supply from van to operate the electronics or they won't run, this is not for making it cold though just to keep the fridge brain running.