hotspot telenet wifree

Hi,

I have problem to signing in to telenet wifree over here in Belgium. Problem is with my macbook and my imac. Telenet wifree is a hotpsot service. Problem is not on android or windows. I called the support from telenet. Everything is working over there. They say it has something to do that apple don't trust or correctly sign in the certificate that is necessary to connect with telenet wifree. I have os x el capitan 10.11.4

Anyone a suggestion to fix this problem?

Thanks