Checked the forum and manuals for Grandstream. It says we can download certificate from “lets encrypt” but checking the forums here, i cant see any Grandstream related issues registered here by anyone.
Normally, you install that client on the machine you want to use the certificate.
So the client
creates a certificate
and is able to install the certificate
You can install a client on another machine, only create a certificate, copy it manual to your device and install it.
That's possible, but it's painful.
So normally such special devices should have integrated (and own) Letsencrypt clients. If yes, that's always the preferred solution (Fritz Box, Synology DSM, a lot of other products).