Sub domain multisite Let’s Encrypt certificate not found

For some reason my Let’s Encrypt certificate ran out and now I have the same problems getting things working again as when I started (and have been doing so for 5 days now…).

My host uses Let’s Encrypt. I just click ‘activate SSL’ and a few seconds later I’ve got a certificate. It works on the main domain, but not on the subdomains such as this one.

For some reason Let’s Encrypt won’t just give me a wildcard. Previously I have tried to ‘fool it’ using a dummy real subdomain, but wildcard still wasn’t checked. The host did something “manually” (without saying what) which worked, but now a few years later the servicedesk doesn’t seem to understand the question.

On another multisite with subsites (not subdomains, even a mapped one) I use Really Simple SSL, but when I try it on the subsite network, the plugin keeps telling me there’s no certificate. I’ve tried a whole range of htaccess edits, functions.php, the plugin, combinations between the previous. No succes so far.

Suggestions are welcome!