Install a CA Root Certificate on Android
Installing the Root Certificate you got from this site on Android, this one: ca.ssl.indexnl.com.crt, is easy.
Open Chrome and just click the link above, and the following dialog pops up:
Give the certificate a name, and its done.
If you use Firefox, you have to repeat the procedure with that browser, because it keeps its own certificate store.
Unfortunately Android requires some sort of authorisation at the logon screen, like fingerprint or pin code, before you can install and use your own installed root certificates.