How to Disable on Iphone and MAC OSX Clients use of Certificates

Hi, I use on Bufallo Router the Freeradius Server future and all works well with Certificates, but sometimes after router reboot some clients can’t login in the Network if Certificate used. If I disable the certificate check on windows I can login without Certificate but on new Macbook and Iphone I can’t disable Certificate check. Has anyone idea what I can do? and is this dangerous to communicate in Wlan without Certificate check activated? Can the Wlan without use of Certificate easy be hacked? Best Regards Nedi -