Changing the Experiabox doesn't help with wifi-problems at all if there are Superwifi's or Experiawifi's in use. These take over the wifi from the modem and the wifi on the modem should be turned off automatically.

Sometimes this doesn't happen automatically though and then this disturbs the mesh-network that is created by the access points. Can you check if the wifi is turned on and turn it off if that is the case?

Do you have a way to check with a wired connection? If the speed is off there as well then there might be more going on.

Hi NHendriks thanks for the answer

speed via cable ~3-14 Mbit (I use DVS) 

sometimes the values change. up or down.
  but it is impossible to use.  the Internet pages do not open  but at the same time Fritz show me WIFI 100-120Mbit, https://www.speedtest.net/  show  1-3 Mbit or 0
frequently I get pop-up  "no internet connection" 

this is the best speed in the last 3 days. 
wifi ~10mbit and Lan ~26mbit 

where to turn off the wifi? on Experia? how to do it? there is always a green light on the internet icon If a cable is inserted.

There's your problem. DVS or any other type of powerline adapters are very receptible to interference by switching power supplies, LED's and other energy saving equipment. Can you unplug all the DVS’s and plug a wired device directly into the modem and test again.

What type of Experiabox do you have? They all work differently. Easiest is to log-in to http://192.168.2.254 and use the standard password on the back of the modem, then you'll be redirected to the admin-interface where you can disable wifi.

How are your Superwifi's connected, one directly in the modem and the other wireless?

Also, Speedtest.net says your provider is ‘Colt’, it should say KPN. When I check the IP blocks owned by KPN these aren't there. Is there a VPN active when testing?