Ipad-How to convert AVI Videos to MP4 off a memory card

I am making videos on a DVR which captures AVI format videos on a memory card.

I have an adapter for the memory card to access the videos on my iPad. I need to find step by step instructions to convert these files into MP4 format and save in my files to be attached to documents later or to save to Dropbox, etc.

I have tried several apps and online converters, but haven’t found anything that works consistently, efficiently and easily.

Any suggestions would be greatly appreciated!