Hi @SKL50 

Thanks for posting in our business forum.

You can refer to this:

What If My Windows Computer Is Not Accessible or Pingable Over the VPN/VLAN In

This is a more of a problem with your computer/device firewall settings. Not a router level issue if you have not configured any stateful ACL.

Hi @SKL50 

SKL50 wrote

  @Clive_A Thank you for your answer, but it's not Windows related. I actually don't own a windows PC. 

I have the issue with synology, homematic, reolink hubs/gateways. All not pingable interestingly a wired Mac is pimegable from the other wireless network. 

This provides you a lane to think about the firewall if it is the reason for the problem.

I have not had a problem with the VLAN interface for years after it was added.

If you connect the computer A and B to the router directly and disable the firewall, do they ping?

I don't know what device or OS you have, nor how to disable their ICMP firewall if there is one. You need to refer to the OS manual. But the truth is, a VLAN interface by default without ACL config, allows pings.

A cellphone can be used to test as well to prove what I write. If you want a demo, I can get you a video.