Ah, okay, I understand. You’ll need Qos (Quality of Service) or ‘traffic shaping’:
[OpenWrt Wiki] QoS (Network Traffic Control).
This is unfortunately on my todo-list, I have no experience at this topic, now.
I know I’ve seen a button ‘priority’ in the GL.iNet app. I think on the ‘Clients’ page. But this would be only a little bit of what QoS is.