Here’s a video on how to save maps for off-line use. This is great when you want fast, detailed imagery for an area you are going to.
The resulting map display is much faster, even if you have Wi-Fi access when doing the imagery, its awesome when you have cellphone-only service, and indespensible if you have neither. This works for any online map source ATAK can use.
You are best off doing this in Wi-Fi coverage.