Wi Fi issues. All devices connect to the wifi except my MacBook Air

Hi there,

I need some help with wifi at my folks house. This has been going on for a couple of weeks and it's a total mistery for me.

Network diagnostics says "Your internet connections seems to be working correctly" .

I see the wifi sign connected at full with all lines, but I can´t surf the internet.

I tried

Restarting the modem

Restarting my computer

Deleting the network from my computer

Calling the tech service from the internet provider and they have no answers (as my phone and my dad's computer seem to work fine).

Can somebody help me?

Gracias!

GEMA