I'm getting "Call to undefined function where()"

I don't know your experience level - but you are doing some very complicated database calls - and it sounds like you are not certain about your server.

So what i would suggest is to do some step by step sanity checks - do some pages and models where you are doing much simpler database calls. Make sure that all works on your server. If there are still issues then just get another host because PHP hosting is really cheap especially compared to your time.

Otherwise if its not the server then build up some of your functionality with some simpler models and db calls. And seek out codeigniter tutorials and libraries so you can increase your knowledge. Then go for the more complicated queries.