I believe you can open dgi go, go to the map view, scroll to where you're flying and load the maps. Don't close the app and then go to where you planned to fly.
I may have done that once (I have a dedicated iPad 5), but I don't recall if it worked.
Fwiw, I picked up a Huawei ascend xt2 android phone for $70 at Walmart (with no cell plan). The android version of go still has offline maps and this an amazingly good phone for the $. My plan is to use the phone when I'm away from wifi.
It amazes me that DJI has not done a better job of dealing with offline maps. The solution as described earlier in this thread is cumbersome at best. Most of my flights take place in offline areas. I would nice to be able to choose Google Maps as my offline maps. I most often use Litchi to fly in these areas. Litchi uses Google Maps. I have several areas that I often fly in cached on my iPad mini. Many tablets do not have an on board GPS receiver. My iPad mini does not, so I use a bluetooth GPS receiver to solve this problem.