How to point on-prem app on domain-name with public IP

Job ID: 32261736

Budget: ₹750 – ₹1,250 INR

I have mydomainnames.com pointed to my on-premise public-ipv4 address.
There are several web apps/servers in my internal LAN therefore the modem should refer to internal DNS server to resolve all domain-name queries. Internally (on-prem LAN) everything is working fine. But I want mydomainnames.com from public networks to point to right server inside my LAN.
Am I missing something? Do I need to point port 80 to DNS server? in "transmission > NAT " section? Any routing policy I should consider?
Help me setup my TP-Link modem so that it should refer to the DNS server (Windows Server 2019) and loads my sites from on-prem server.
Thank you.