How to access music from hdd through phone without PC

I have over 600gb of songs i would like to be able to put on an external hdd and have them play through my ipod or phone (ipod preferably). I need for it to work without the use of wifi or mobile data. I also wont be able to plug it into 240v power, it will need to be battery powered or charged via 12v connection. Im trying to work out how to have all my music with me while im camping in remote areas with no mobile network range.
I am looking at purchasing a Seagate Central 4TB wireless external HDD however there will be no wireless so unless i can access the HDD through my phone without the wireless it will be wasted money.

Ideally the external HDD will be plugged into my phone with an aux cable running from my phone to my jobsite radio or audio player in my 4wd/camper.

I dont see a laptop or tablet being a viable option as i dont think the charge will last long and being a larger device will be more inconvenient.

If this is not possible, any other ideas would be appreciated as running around with 5 phones and a couple of old ipods is a pain in the ***.