Help: Strange question suspecting networking

I have a strange question in my office network; summary setup as follow:

Internet -> Mirkotik -> UBNT AP

I have about 10 PC and 4 servers and a few more devices connected to the same internet connection from my provider with static IP (5.6.7.8).

I have a web server located outside the office at another state, Centos 7 + DirectAdmin, Apche+php.

Locally I have 2 CentOS 6, 1 CentOS 7, and 1 Debian server in my office, recently something very funny happens, 1 of the CentOS 6 server, the Debian server, and 6 of the PC cannot open the web pages from my public webserver at another state.

I tried basic troubleshooting like ping and traceroute to the server IP (1.2.3.4) are fine, can open other ports like my custom SSH / FTP / DA ports. However, with the CURL / Telnet test from that server and those PC to the web server ports 80 & 443, I'm not able to connect or open the web pages from the webserver.

I have also compared the Centos 6 networking setting are the same as the other centos 6 servers which can access. On the web server itself, firewall or DA settings are looking good and in Linux, messages log I also cannot find the "try" call from office 5.6.7.8 when testing with the server cannot open the web page.

Office LAN settings firewall also checked in Mikrotik and UBNT controller, all server settings are the same, I have simple LAN with only 192.168.1.xxx.

Any suggestions or things I shall look into?