My provider offers a CA-certificate in DER format (https://www.ziggo.nl/content/dam/www.ziggo.nl/tools/WifiSpots%20certificaten/WifiSpots%20certificaat%20Android%20en%20Windows%20LGI.der). How can I import or use it?
I’ve solved my problem (by myself). If others should have the same problem this procedure was my solution.
As someone wrote in xdadevelopers forum Android can’t handle .DER/.der certificates. These has to be converted to PEM format. If you’re using Linux and openssl is installed try this:
1 - open a terminal, change to that directory where the DER certificate is located.
2 - Convert the file with the following command
$> openssl x509 -inform DER -outform PEM -in YourWifiCertificate.der -out YourPrefferedFilename.crt
3 - Connect your FP to the computer, activate the USB storage mode.
4 - Copy/move the converted file with CRT extension to your FP.
5 - Disconnect/unmount the FP from your computer and deactivate the USB storage mode
6 - Use a filemanager (I use X-plore) and change to that directory where the converted CRT certificate is copied to.
7 - Tip on that file, Android will automatically detects this file as a certificate which can be imported.
8 - Import it
9 - now open WiFi configure screen (–> settings --> Wifi)
10 - long tip the Wifi-network for which you need this certificate and select 'change’
11 - Scroll down to the end and enable extended settings
12 - Scroll further down to CA-Certifcate and select “(nothing)” or what’s written there
13 - Now you can see the usable certificate. Select it, name it and select save
14 - If you currently use no screen lock you have to make one otherwise the certificate will not work.
15 - That’s all. 
The openssl command is derived from android.stackexchange.com, the the 3rd post.
Hope, it helps.
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