I have tried to set 2 A records for my domain voquangphu.org.uk but when I ping the domain it always returns only one IP. Do I need to turn the feature on explicitly somewhere or wait for DNS to update the record accordingly or maybe it was because I set it up wrong somehow?

Thanks for answering my question.

You do not need to turn on round robin feature. If you have more than 2 A records set up for your domain name in the control panel, the domain name will automatically resolve to 2 A records.

Thanks for answering my question.

As it turns out there was a problem with IP caching of my computer (using MacOS) that causes only one IP to be returned. After flushing the cache, the 2 IPs were returned in round robin fashion successfully.