Thanks for starting this topic @MartinGabriel! I checked your connection right away. First of all I can see that we deliver a maximum speed of 36Mbps on your address, so 50Mbps will not be possible to reach. That being said, your connection looks great. The speed you measure, does this concern wifi or ethernet? Please know that we only guarantee this 36Mbps connection through etnernet. Wireless, the connection can fluctuate. If you do measure this 18Mbps by cable and you tried the basics (testing another device, another UTP cable, resetting your modem), I’d like to receive a couple of screenshots of speed tests. Please take a close look at the following steps! If the tests don’t meet the necessary criteria, we can’t start an investigation.

-Go to www.speedtest.net/apps in your browser and download the app for the device on which you want to perform the speed test
- Please shut down all open systems including VPN
-The computer or laptop has to be connected via an ethernet cable, a speed test of  a TV or gaming computer can’t be used
-Run the speed test. For the best results it’s best to test on several target servers. When you press the big GA-button, you can choose a server. Is the speed below the attainable average? Repeat the test on the same device but with a five minute intermission between the tests
-Take at least two screenshots of speed tests on different target servers and make sure your whole desktop screen is visible, so we can see the connection type (it's important that it's visible that the speedtest is made with a internet connection via a ethernet cable)
-Take a screenshot of the taakbeheer (Task Manager, on Windows) or the activiteitenmonitor (Activity Monitor on iOS):
In Windows you’ll find this by pressing CTRL + ALT + DEL; choose more details if they’re not directly visible
In Apple MacOS you can search for "Activity Monitor" from the Spotlight search box 
-Take a screenshot of the IP configuration:
Windows: via the search function under shortcut “Windows + R”, type in: “cmd” and press Enter (or search for Opdrachtprompt/Command Prompt in the starting menu). After opening, type in: ipconfig / all and press Enter
Apple MacOS: via the Finder or the Spotlight search window you can open the Terminal app. Type in: /sbin /ifconfig and press Enter. This will display all connections the device has at that specific moment. 

Please send me the screenshots and make sure that the mentioned steps are followed precisely. Thanks in advance!

Dear Cheyenne,

I did follow all the septs above also checked and change the cable for both the DSL from the socket to the modem and the Ethernet from the modem to my PC. I have tested the connection on multiple devices, (my laptop, phone and my wifes phone) and across all the devices its showing the same results through cable as well as wifi. 
This gives me the feeling that its something between the modem and the connection to the house. its good to mention that there are two apartments on this address but both have separate internet contracts and providers. I dont know anything about the technicalities of the connection to the network. But in my memory I was getting at least 40 mbs with the pervious connection from KPN. I measured this years ago so I cannot tell exactly if this is recent situation or its been like this for some time tho the internet feels a bit slower now. Is it possible that this would be caused by the connection in the house ( like bad wiring somewhere or something like that?) 

Thank you for the help and I hope we can resolve this somehow. 

Thank you so much for all your work @MartinGabriel! I have checked everything and discussed the situation with my colleague. We decided to send a techician your way to investigate on location and fix the issue. Guidion will contact you to make an appointment. Thank you for your patience, this will be fixed as soon as possible!