@RvS2020 problem solved. Deco's were installed in router modus, but when 1 of them was set into accesspoint modus, the problem was solved. Helpdesk TP-link helped me. Thnx

Since installing my Deco M9 plus its hard to call with whatsapp (data-call) to another user of the Deco M9. It takes 20-30 seconds to get connected on wifi. Phones are put on wifi-calls possible....