I can't install SSL cert in linux server so is there anyway that if I install SSL cert in mikrotik and it'll make the connection HTTPS??You need SSL certificate on device, which terminates connections. For HTTPS it's web service (or reverse proxy if one is used), not the router performing NAT. So even for HTTPS you only have to port forward external TCP port 443 to your internal server (preferably port 443 as well) which will handle SSL just fine.