WP multisite and Let’s Encrypt SSL certs

I have a multisite with mapped domains. Per WordPress instructions, the mapped domains are parked in the main site’s server, with their A records pointing to the server.

The auto-renew for the certs failed recently. I’ve been advised that, in order for auto-renew to work properly, I need to use the same nameservers as the main site, that just pointing the A record to the main site’s server won’t work. This essentially means the mapped domains are made to be addon domains on the main site’s server, not just parked domains.

(1) Does anyone know if the above (ie about nameservers) is correct ….

(2) … and, therefore, I should addon (not just park) the mapped domains

(3) And how do I set it up as an addon domain so subsite.mainsite.com (a WP site) works.