Thanks for bringing this question to the community!
While I have no experience with onX maps, I am a basic level user of Gaia, which is used by @Philreedshikes. Also, @Gary2 recommends the Guthook app. I'm tagging them here to see if they have some insight into what it takes to lay a route down on a map and use it when the phone is in airplane mode. @Wanderer and @CajunHiker may have some experience to share as well.
You are able to use the GPS on your phone when it's in 'airplane mode' and/or unable to receive a signal from a cell tower. You should be able, when still connected to wifi, lay a route on a map that you can then use in the field while not on wifi/cell network. You can read more about that on the onX app here. Additionally, here is a piece about creating routes on your basemap on the onX app.
Hopefully this helps, thanks!