rootca.pem worked for me. Had issues after the "Let's Encrypt root cert expiring" fiasco.
Spend 2+ hours troubleshooting. Fixed in 2 min after reading this.
Edit adding more detail ---------------------------------------------------
Followed goliash's post, except used WinSCP or MobaXterm to SFTP to my QNAP. Drag/Dropped the newly downloaded rootca.pem and overwrote /etc/ssl/certs/rootca.pem
If you encounter failed to install or renew myQNAPcloud or Let's Encrypt SSL certificate on QTS 5.0.0.1785 build 20210908, this can be resolved in subsequent QTS version, or can using the steps below to fix manually:
1. download the rootca.pem file and upload it to the NAS Public folder on your NAS.
2. connect to the NAS via ssh (steps to access ssh can be found here).
3. issue the below command to overwrite the original rootca.pem file on your NAS:
cp /share/Public/rootca.pem /etc/ssl/certs/
Once this is done, you should be able to install the SSL certificate successfully.