It probably won't be the most elegant solution to this situation. But I was stuck with Adguard Home in a Docker setup and desperately in need for a SSL certificate.
Fiddled around some, but got stuck even more in my newly found time sink. After finding the thoroughly written blog posts  and  of Martin Falkus, it put my train of thoughts in motion. Thanks Martin!
- Disclaimer: you need to be on at least DSM7.0
- Set the hostname in Control Panel > External Access > Advanced
- Go to Control Panel > Security > Certificate > Request a Lets Encrypt cert
- Create a new action in Task Scheduler:
- _archive because it needs the actual files, not the symlinks in the live folder
- the ppUPmS directory will differ, it's just a random identifier which luckily remains the same even after renewing the certs
Update your yaml file or container config accordingly so you can link to the certs:
And now the fun part. You can go update your setting.
The task scheduler script runs every night, and I requested the cert just before midnight. So basically there will be a couple of minutes every 90-ish days Adguard Home won't have valid certs to work with. For me very workable, not the most elegant like I said but hey.. if it works it works.
Let me know in the comments if you perhaps have any suggestions to make it more elegant 🎩😉