Thank you for your help.
It wasn't a Plesk issue after all. It was my dumb mistake. I misunderstood the modsecurity's whitelist syntax.
I thought by adding id:210280 that it would whitelist the IP for that particular rule's ID instead of being just the whitelist rule's ID.
Everything works fine...
My server configuration is Plesk Onyx 17.8.11 on Debian 9.5 with Comodo ModSecurity (subscription).
When trying to whitelist one IP serverwide in ModSecurity with a rule such as:
SecRule REMOTE_ADDR "^192\.168\.1\.1$" phase:1,id:210280,nolog,allow
It throws the following error:
"Invalid...