Is there a base tag on the page? It seems that you are using relative urls and with routing and no base tag it tries to load the content relative to the path in the url and not where the page is located.
Then on a Ctrl-F5, the routed page shows as '/admin/business/3' highlighted with green box below.
And all the 404 pages have added the '/admin' directory as highlighted with blue box.
The '/admin' directory is where the actual page sits 'https://daviesandgray.co.uk/admin/_manage_business.php'.
The Wappler files are actually stored at root level as shown above, but because they have '/admin' added to them, then they cannot be found.
I see on your screenshots that the base tag is at around line 90, it should be about the first thing in the head. Any script tag above the base tag will not load correctly if the base comes later.