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I think the IP certificate you mentioned is actually dedicated IP…
In short, no you don’t need an IP certificate…
You could use an dedicated IP to avoid some older clients who don’t support SNI (server name indication) and that’s mainly all the difference between dedicated IP & shared.
We can’t control your host’s policies or interpretations, but I agree with @stevenzhu. It sounds like your host has either misunderstood or misstated something (perhaps an issue about SNI and old browsers), or else simply wants to charge you extra to allow you to use Let’s Encrypt certificates.
The problem is that https://www.sillyoldsod.com/ (which has a working certificate) redirects all connections to https://sillyoldsod.com/. The certificate presented only covers www.sillyoldsod.com, not the base domain name sillyoldsod.com. The certificate should most likely be reissued to cover both names.
A separate certificate for the base name sillyoldsod.com (by itself) was issued yesterday