I'm not aware of any dictation software available for the iPad that will work offline. Voice recognition on the scale needed for dictation takes a lot of memory and computing power.
My guess is that it would have been difficult to make it work reliabley with iPads before the iPad Air 2 and iPad Pro, and that no one has gotten around to making an offline verison just for the newer modles.
Keep and eye on Dragon Dictaton by Nuance. They are the most likely to add this feature to their app. You could probably find a way to give them some feed back on thier support site, if you want to make sure they are considering the idea. (No idea, I havent looked at their site in ages).
The only solution I can think of that will work is for you to use cellular data with your iPad. If the iPad itself is not capable of it, then you could use your phones hotspot (if available) or purchase a Mi-Fi or other portable hotspot solution.
I'm sure that's not what you want to hear, but if you have to have portable dication on your iPad right now, then that's the only solution I know of.
Good luck.
