When your ground station shows link-timeout (RED) after 5 seconds, then you can also not send any new command, flightplan, setting, ... to the plane...

When you have LEDs on your modem you can clearly see that as long as LINK is green, a flightplan command (e.g. standby) is being sent, but as soon as LINK goes RED, it does not send anything anymore.

Surely this is not intended? Do you know if this is a LINK / SERVER / GCS issue