I have a simple query as 'how to point domain names to on-premise web-apps?'

Here's my setup: 

Internally my DNS server points all domain names properly to its respective apps/servers/ports. But I am not sure how to direct external traffic (outside LAN) to point to proper internal apps. 

<fixed-public-ipv4> points to <Modem> 

[A] Modem Configs: 

1. Modem IP 192.168.0.1 subnet:/24

2. Modem> Network > LAN > DHCP server > 

 Primary DNS server set to <192.168.0.2> (WS 2k19 DNS server)

3. Transmission > NAT 

random port for VPN (works fine) ponts to internal OpenVPN server 

nothing else.

therefore port 80 on <public-ipv4> gives not found error. (that's fine). 

[B] DNS Server configs: 

DNS server has several forwarding DNS for zones such as 

mysite1.myorg.local --> .0.31
mysite2.myorg.local --> .0.32
mysite3.myorg.local --> .0.33
 

also public domains

myorgsite.com --> .0.34
testing.myorgsite.com --> .0.35

etc. 
[C] A records of DNS from domain registrar points to <public-ipv4> of my modem.  

[D]Desired outcome:

The websites domain names reaches my modem but dont load the proper webserver/app from on-premise LAN. Although I have pointed my primarly DNS as internal local DNS server. 
I need myorgsite.com from outside query to load server/app from .0.34

What is missing? Do I need to add something in "tranmission" section? NAT, routing pages?

This seems simple and obvious but I do not have a how-to guide on it. 

Thank you