Hmm.. Interesting.. Can you define what you mean by "online" ?? 

Obviously .. You can access your iCloud databases from your Mac or your iPad / iPhone.. 

You can also use remote control software to remote into your mac from another machine and access it.. 

Via another server / web app... Hmm.. doubtful.. but have not really thought about it that much. 

May I ask why? 

@Mr. K.. 

OK.. Hmm..   While I suspect "anyting is possible"... 

I would think you would use the Ninox Cloud .. and then create an API key and access your database via the API ..   Lots of people do this with various other "form" type web applications .. as well as use Integromat (or that Zthing..) to bind their Ninox database with other applications. 

Ninox cloud account.. Works via browser ... There are a few differences between cloud and mac app.. but all in all.. works great!

Ninox do a single user standalone one time purchase version for mac and icloud sync (which i think you can turn the link off) at £34.99

@Mr K.. from my knowledge .... if  you want to view it via a web browser, you need to add the Ninox cloud subscription. 

If you want to access your Mac App via another computer .. use a remote control application .. like Jump Desktop or your favorite flavor. 

Until today I had no problems with the database app (online)