Changed to HTTPS and now site locked out for visitors & admin!

I added the ‘really simple SSL’ plugin to the site and changed the WordPress Address and Site address to ‘https’.
Now all visitors and admin are locked out ;(
Now the home page asks visitors to “Sign in” with UserName & password.

I found the thread:
https://ww.wp.xz.cn/support/article/changing-the-site-url/#changing-the-url-directly-in-the-database
and followed the ‘phpMyAdmin’ instructions to change the ‘site url’ and ‘home’ values from “https” to “http”.

I have done this*, but the site still still asks for “Sign in” credentials.

* and cleared the apache cache … and tries again several hrs later, and on different devices ;(

Where/what should I do next?

NB the host is lcn.com who (I think) provide multi-site WordPress – so (I think) FTP access/changes will not work (please confirm)
Thanks