5Ghz wi-fi network airport time capsule

I'm connected with my MBP to my APTC via Wi-Fi.

The APTC is connected to my Internet box with an ethernet cable.

When I' m starting my MBP and I'm connecting on the 5 Ghz Wi-Fi network each time I have to switch to the 2.4 Ghz and coming back to the 5 Ghz again to have a quite good network speed.

Before I change the speed is less than about 200 Mbit/s and after the switching between the networks the speed growth to about 800 Mbit/s.

Has anyone already met this type of issue or every idea is welcome.