I have been having trouble accessing a particular web address ever since I downloaded their software to run 2 years ago. I wish to upgrade now to their paid version because I like it so much, but to my anguish my attempts to connect always seem to time out, and ping tests prove this. Their IP address is not recognised as a valid address, yet when I use my mobile I can access their website. I can also access their website using others devices. My children when they come home or visit at my office cannot access the site with their laptops, but can when we leave the office or home. It often states that I must connect to the internet to access help from its software options, but I am connected. I have screenshots to prove this.

I have checked with my service provider and they say they do not block that site, and the website administrator also confirms that they are not blocking me.

I downloaded anonymizser to run one my home pc and this enabled me to connect to the site so it is my internet ip address that is the problem.

I have checked all my router and computer firewalls, and I don't have any.

I run AVG 2013 ANTIVIRUS on all machines with their firewalls deactivated.
AVG have looked at it and so far cannot figure it out.

My IT consultant cannot figure it out.

I am at a loss what to do next.