Hello everybody!

I'm the proud owner of a RB951G-2HnD with RouterOS 6.7 and firmware 3.10. The device is configured as a Home AP with DHCP WAN (my ISP provides me with an IP based on my MAC address) and a DHCP server for LAN in which I have 3 computers and two smartphones all receiving IPs from the router. So far, so good... but there's a problem I really can't explain and I'm wondering if anyone else has met this issue or have any idea that might point me in the right direction because I don't know what else to do.
My problem is that I cannot access specific sites from two devices at the same time. So if a computer established a connection to a site, the other devices cannot access the same website even thou the website replays to ping. I also did a nmap scan of the site and the computer that had the initial connection to the site showed open ports, but the second computer didn't see anything. If I use Tor, or a proxy the website is available from every computer.
I have tried: resetting the router, downgrading the RouterOS and firmware, changing MTU, changing browsers, changing OS on computer...
Am I doing something wrong ? Has anyone met this behavior before? Any ideas?

Thank you!