menu items direct to 404 on wrong sub domain

I am just learning how to use WordPress. Am not technical, don’t understand code, so please be patient with me.

Main site is https://www.schoolcouncils.org

Have just had the chap who hosts our sites set me up with two subdomains. They are:
https://primary.schoolcouncils.org/wordpress/ [using theme Rainbow]
https://secondary.schoolcouncils.org/wordpress/ [using theme Mobius]

I suddenly have a problem with secondary.schoolcouncils.org/wordpress.

When you select any item from a sub menu to read it, it takes you to primary site and gives you a 404 page. The link is correct, and although it actually says in the web address bar that it’s showing you the correct page, it actually isn’t.

I have deactivated the plugins I have installed and refreshed the site and problem is still there, so I have reactivated them.

I have been through and updated all the pages and the problem is still there.

The Primary site works fine.

Please can anyone advise me, any suggestions gratefully received.

Thank you.