Switched to Google Site Kit, now analytics no longer working

I originally connected my site to GA using a plugin called ‘GA Google Analytics’, which worked well for several years. However, recently I decided to replace this with Google Site Kit. I deactivated the original plugin, installed Site Kit and set it up. I confirmed that the same id was being used as before. Nothing came through to GA. So I deactivated Site Kit, reactivated the original plugin – and still nothing. Now neither plugin will send data to GA. I’m not very technical; I used Chrome’s Tag Assistant, which shows the tag with the correct id but says “no hits were sent by this tag”. I also connected Site Kit to my Ads account (the original reason for switching to Site Kit), following the instructions on the Ads site for Site Kit, at the bottom of which is a ‘test your website’ option. This returns “Your Google tag wasn’t detected on your website.”

The Site Kit dashboard shows historical traffic i.e. up to the point when I switched over, so it is correctly connected to GA. It’s also connected PageSpeed, and showing correct data from that on the dashboard.

The page I need help with: [log in to see the link]