Cannot access website

I am trying to connect to a website bpo.legalgard.com/lma and all of my workstations return with the error “site not found”. We are dialing in using ISDN with a SonicWall firewall. We are able to browse any other websites. At another one of our locations we are able to connect to the site and we have DSL connectivity with the same SonicWall firewall. We are using two different internet access providers. I changed the DNS settings to the ISP DNS address that was working and we were able to log into the site. However, 8 hours later we were unable to login to the site again. At our second location we have no problems.