encounter issue with our website

Hello All,

my domain name and website is same name , users cannot access website from internal network, but is  working well from external network.

My website hosted externally.

my solution : I created A record on internal DNS pointing to IP address for website, this allow users access website but very slow and cannot see the pictures in website (not loaded correctly).

Note: when set Public DNS like(8.8.8.8) to LAN interface everything working fine.

How to solve this issue please?

Thanks