also: an IP cannot have an letsencrypt ssl-certificate. at least that was their policy for years. so you’d need a (sub-)domain for your host anyway.
this means your IP should be set correctly as A entry for the domain name you are using as hostname. and this domain should be set as hostname on your linux.
it does not mean to use your IP as hostname - that would be kind of the opposite
usually when you have the domain set as hostname before you install hestia it will pick that up automatically and create a web-domain entry under the admin-account for it.
so easiest would be to add the new one there (as web-domain) in the panel and enable ssl/LE for it. after you got the cert you can use the v-host commands to tell hestia to use this one for the panel…