Problems transferring to a new host

I’m having real difficulty transferring my site to another host. I had built the site here:

https://www.tetedelacourse.co.uk/

and I want to host it here:

https://hillsidehosting3.co.uk/fopp/wordpress/

I’ve transferred all the files to the new server and exported the database and imported it to a new database on the new server with a new user( with all privileges) and you’ll see by clicking the new host link that it has sort of worked.

I’ve then gone into my test site the original one) and changed the URLs to the new site (as per the instructions on “If You Don’t Care If Your Old Blog Works” https://codex.www.ads-software.com/Moving_WordPress) which immediately logged me out and wouldnt let me back in. I googled this and came across https://codex.www.ads-software.com/Changing_The_Site_URL

I’ve edited the functions.php back to the original test url with the result being that I can access the admin of the test site again and the homepage still works but clicking on the store pages brings up a 500 internal server error.

I had backed up both the test site files and the test site database before doing anything so I think it can all be retrieved but I’m pretty terrified to do anything else that might further mess up everything I’ve done so far.

Any help anyone can give would be really, really appreciated.