Issue with a government site where i need to use digital signature

I am in Denmark and would like to get access to https://sede.mir.gob.es/opencms/export/sites/default/es/inicio/ when I press Access throug digital signature, It says secure connection failed. To be able to access spanish government sites with my spanish digital signature how to get firefox to approve it? I was tried to add the websites and approved but didn't work. The certificat is installed correctly and tested when I was in Spain. Thank you for your support.