@David-TP You may.  Why I want to do this is not really needed to answer the question, but sure. There are serveral reasons, some general and some specific.

In general: it is my router and it has a firewall in it. I bought 4 units so it was not a small investment. I own them. So I should be able to view and change that firewall.

A little more specific: I am responsible for the security of my home network. I have an extensive network with a good deal of sensitive, priviledge and proprietery information on it. It would be irresposible of me to blindly relay on a for-profit coorporation based in another country to provide security for me, regardless of how good your security may be.

And to the specific use cases:

The first case I ran into was needing to open a port 5938 for Teamviewer.  I have at the moment three computers running Teamviewer. Port forwarding only allows me to forward a port from one machine, it does not open that port. See from Teamviewer:

"TeamViewer prefers to make outbound TCP and UDP connections over port 5938 – this is the primary port it uses, and TeamViewer performs best using this port" 

This is just the first instance that I have run into, but this applies to many many scenarios. Home automation services and endpoints, Port 3389 for RDP on both Linux and Windows machines, port 5901 for VNC on Ubuntu, other services like ssh and sftp, AND.... while I do not play video games, I understand that some video games use dedicated ports, and I would be very surprised if there are not many households that have more than one kid playing the same game.

Feel free to ask any other question, but please also just tell me how I can do this.

Thanks

  @David-TP  Oh! I forgot.

There are my OBiTALK VoIP phones.
From OBiTALK:

What ports should I keep open on my router/firewall?

In order for your OBi to be able to send packets w/o interruption, please configure your router as follows:

Allow Outgoing:
TCP Ports: 6800, 5222, 5223
UDP Ports: 5060, 5061, 10000 to 11000, 16600 to 16998, 19305
Allow Incoming on UDP Port: 10000

  @David-TP 

Just purchased the XE75 3pack and so far I see a laundry list of disappointments. (Coming from a Synology).

I too very much require to block certain outgoing connections to the internet on specific ports. 

Why is this not available? It's a basic functionality IMO. 

Can TP-Link please add this feature?

Thx

  @HHaas 

Sounds like you guys bought a consumer based system expecting professional features.  I totally get what you are trying to do and why you may have thge need.  Just sounds like you purchased the incorrect product.  I would suggest putting something like a Firewalla in the chain or take back the Deco and purchase something more ProSumer or Professional if you want the control.

  @Cherk947 

Sounds like it. I returned it... Going back to the Synology family of devices.

  @MartinXE75 

Good luck my friend.  I came from the Google Nest system and the XE-75 blows it away.  Getting 600 MB on wifi!

  @Cherk947 

I get close to 500 Mbps on a 3-4 yr old Synology Wifi5 one... So 600 wasn't much of an upgrade and then all options I lost.

  @Cherk947 You say this, but even my default router from my ISP has a basic firewall on it allowing me to block inbound/outbound traffic as required. I wouldn't say its out of reach for a consumer product to offer this feature.