Wi-Fi won't work when too close to router

My MacBook Air says: "Warning: No internet connection" when too close to my router, but it does not disconnect from the router.
When I walk into the kitchen, away from router, the internet connection is back.

I tracked down the beacons and there are no lost beacons/packages etc, as I heard that beacon loss happens when too close to a router. But it looks as if everything is normal.

Note that my Asus and Samsung connects anywhere in the house with no problem.

Thanks in advance.

is the aerial pointing upwards on the router

yes...

There are two antennas, both pointed up.
Pointing antennas towards the wall seems to be a temporary solution as the internet works for about 5 minutes.

So what now?

Access your router's internal menu (see your manual to learn how) and change the channel it's broadcasting on. Also if your router is a dual frequency model, change to the other frequency. (2.4 GHz vs 5 GHz or vice versa)

try moving the router or putting it on a different place. The wireless signal may also be degraded by dealing with partitions or any other solid objects.