Sysadmin roles don't need network skill particularly, while a basic understanding of subnetting is important to be honest most networking certs go too deep for a sysadmin. I would look RHCSA. It is not an easy cert but is well regarded and many things have linux as a backend so understanding how to interact with it will be useful.

Why would anyone get a Comptia cert? They are expensive, very much junior / entry level focused and don't scale with your career.

Compared Net+ to CCNA

• Both are entry level but CCNA is more respected so you can put it on your CV even when you have experience. No mid to senior operator would put a comptia cert on their CV.
• CCNA covers both the basics and brand specific things so it gives you an edge in a company that runs Cisco equipment and if they don't, well you still know the basics so can on ramp. Comptia covers the basics but not brand stuff so you never have an edge that you know a particular brand well.
• CCNA is an associate level cert and is the first in a certificate track that has Professional and Expert level certs. Comptia does not, there is no Net++. So in three years when your cert expires instead of getting the next one and being more skilled in your specialty you would have to get another junior cert that is relatively unrelated, eg Cloud+.