How to troubleshoot and fix website's timeout error?

I have a big nightmare on my head.

I rented a shared hosting server and provide hosting for a few clients. The IP of the server is 107.6.169.82 and it is obviously shared between the hosted sites, 4 to be more precise (onemoreframe.net is the main issue).

The big problem is that some people can't access any of the sites hosted there. The hosting company told me no IP is blocked. I also checked the cPanel and WordPress back-end for blacklisted IP's. There were none.

The hosting people keep telling me that it's a service provider thing and keep telling me to reset my router and clear my cache, but obviously, without any result. Plus, it's kinda weird to tell the customers to do that :(

I did tracert and ping on the server IP from one of the computers with the problem. They were both successful but the website wouldn't open in the browser and gave a timeout error.

The issues appear to be mainly in Bulgaria, where my clients live, but might be general as i have no way of knowing and all the online tests I did were successful.

I was thinking it might be an ISP problem, with them blocking some IP's. But i tested the website on 2 separate PC's with the same ISP. On one was working, one was not.

My second best guess is something to do with the DNS, but as i am not that brilliant, i don't know how to test or even if it is possible.

Can one of you lovely people help a poor tormented soul? Thanks a lot and have a great day :)