@alepapadop 

Usually, the client will not show up on the router's DHCP list if you set up a static IP address on it, it just shows up the clients with dynamic IP.

Even though you can not see the module on the router, it should be working, does it work? Can you ping through the module's IP address on your computer? You can refer to the instruction below to ping. 

https://www.tp-link.com/support/faq/425/

@Stella.Q

I can ping the alarm module and also I can access the web interface from the LAN, but I cannot connect from the WAN (i mean via the internet), the router is rejecting any connection. I have created a "Virtual Server" an "Address Reservation" and enabled the "DMZ" for the alarm module in the tplink router settings but I had no luck. Well, I think I will be forced to reconnect my old thomson router as a modem and use the tplink only as a access point. This is so disapointing. What is your return policy?

I strongly beleieve a bug report ticked must be created for the issue with the clients. Any device connected to the router is a client and not only the devices that get an ip addresses via dhcp.

@alepapadop 

We can do some troubleshooting about this case. Would you please send us the Virtual Server settings page and the Advanced--Status page?

You mentioned you can access the web interface of the alarm module in LAN, would you please send us a screenshot of it? What's the alarm module's IP address, default gateway and DNS server?

Besides, confirm with the internet service provider and make sure they didn't block that port on their end.

For the warranty policy, you may need to contact the tech support to check.

https://www.tp-link.com/en/support/contact-technical-support/#Hotline-Support 

Good day.

@alepapadop 

Glad to hear that it works.

Regarding the static IP issue, we will feedback to our product engineer and then they will work on and evaluate it.

Good day.