iPhone 6s slow on wifi - ipad and MacBook is fast

I have Fibernet with 50 mb up/down

I use iPhone 6s, iPhone 5S, iPad, MacBookPro, surveillance camera (Y-cam), thermometer (Netatmo).

Everything is workiing fine, except iPhone 6s

When i connect to Wifi i get 1 - 2 mb download. Everytime. On the other devices i get more than 50 mb down.

I have tried to change channel on the Wifi with no result.

Then i got the idea to set the iPhone 6s in flightmode with activated Wifi. Then i got full speed.

When i turned off flightmode, speed was reduced. And i could do it while the speedtest was running and follow the speed on the "speedometer".

Have tried reset of networks and the iPhone. No change.

I have only had the problem with the combination with the iPhone 6s and the Wifi router. Anywhere else the iPhone 6s works fine.

Anybody heard similar problem?

Anyone got ideas for fix?

Best regards

Poul Erik