I am having the same issue with one of my client’s website.
Randomly every few days The navigation menu stops working. It displays the home page fine but as soon as you navigate away to another page it returns a 404. In my case it affects both mobile and desktop.
I found a temorary fix by resetting permalinks but it continues to happen every few days. I believe it could be a plugin issue that is causing a script conflict. I just haven’t found which one yet.
Try disabling plugins one by one (after backing up the site or creating a local copy) and re-enabling them to see if one of them is conflicting.
Hope this helps, if you find another reason as to why this issue is happening I would appreciate any tips.
