Homey Bridge not connecting to wifi

I have just received my Bridge and tried to connect it to my TP Link Deco M9+ wifi. I have changed the wifi settings in Deco to 2.4 GHz only. I have restarted both the router and the Bridge (after 30 minutes).
Everything is according to instructions until I have entered the wifi-password. Get the message “Could not connect…” and the Bridge goes red. Deco supports b/g standard. I do not think it is possible to turn off ac and n.
I have also tried connecting to the hotspot on my phone using the same SSID and pw (all other routers/AP:s turned off). Still not connecting.
What have I missed, or what am I doing wrong?

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One “hack” when 5GHz can’t be switched off during the “add your bridge” procedure:
Connect your phone to your M9+ 5GHz SSID.
Now plug in a long extension cord and grab the bridge.
Walk away from your router, until the 5GHz signal is gone (like, 15 meters or more).
Connect your phone to your M9+ 2.4GHz SSID (which should still be in reach) and enable bluetooth ( → dunno about BT, but Athom says so).
Plug in the bridge, and try to add it to Homey now.