Help with EG 4 18kpw

I am replacing a growatt inverter with a new EG4 18kpw inverter. On the growatt there was only one spot for ac and it provided ac in and ac out, the EG4 has a grid and line. My question is EG4 keeps saying that the grid needs to be between my meter and my Main breaker. But my system is installed on a detached garage where i back feed a breaker there that back feeds my main breaker panel. can i pull the grid and back feed the system from my sub panel? I am assuming i will only lose the ability to have power in the case of a power outage which i am fine with right now.

According to the 18Kpv manual there is an AC In (Grid) and an AC Out (Load) just like the Growatt. The only difference is the 18Kpv also has a second AC Input for a generator which does not have to be used.

Your question is not completely clear, it would be helpful if a drawing or what you are trying to accomplish was included.
1) Yes, you could use the subpanel to back-feed the main panel provided the OCPD and wiring is sized accordingly for the 18Kpv.
2) The grid connection should either be removed from the main panel completely (not advisable) or an interlock plate should be installed so the main breaker and inverter/subpanel back-feed breaker can never be ON at the same time.

EDIT: The last statement is puzzling, what's the point of having an inverter if you lose power in an outage.