Old iPad - how to install apps without other device?
With an old version iTunes on a computer you can download the current version there.  Once you go back to you iPad you can download the older version. 

How to get the version iTunes you need.
https://apple.stackexchange.com/a/320415/44531

The process[ ignore the blurb at the top which says you cannot ]:
https://apple.stackexchange.com/a/144924/44531]2