404 error code and not letting some iPhones get into the site.

I am in need of some help! for the last month I am having trouble with my website after trying out the Duda mobile plugin. I didn’t like the mobile browser look so went back to the desktop version of the browser for mobile phones. But every since I tried out the Duda plugin some users with iPhones have trouble signing up for the site and I have a current member or 2 that can’t log into the site when they could before. I hear it could be coding issue that the Duda plugin could have changed. I was also told that I may have to delete and rebuild my website over so this will stop. It has to be some way to fix this without doing that.