Internet strength

To add to iTinkeralot's comments, you'll need the existing SSID's password to complete the link between the gateway or router, and the extender. I'm not using a router, just the gateway, but the principle is the same. After the link between the gateway/router and the extender is made, you can either replicate the WiFi name, e.g., MyNetwork, or give it a unique name, e.g., MyExtendedNetwork, which is what I did. It just depends on your needs.

As to security, if you are convinced that the credentials for your primary network are secure, you have the same level of security for your extended network. Your neighbors will NOT be able to log on and use it.

If you have the extender connected wirelessly to the network SSID and have provided the authentication credentials then your extended segment should be as secure as your gateway network. The link between the gateway and the extender should behave as a point to point link. If you are connecting to the gateway via an Ethernet cable and are running the same wireless SSID and authentication on the extension device it should be secure. Check the manual for proper configuration of the device you are using to extend the network. 

How do we secure the extension or is it protected by my router security? It doesn't show a padlock and no need of password. I don't want folks in the area to be able to use it?

I agree with ahallasir's assessment. I always have 3-bars and only on 4G LTE. Nonetheless, I can watch two TVs simultaneously and still be able to use the Internet on my MacBook Pro. Forget the numbers and concentrate on performance.

As for an extender, I use a Netgear EX2700 extender for one area of my home that has trouble with WiFi reception. It works perfectly and definitely solved my coverage problem. For the price, you can't beat it! Just one word of caution: Netgear would prefer you to use WPS to program the extender, but unfortunately there is no WPS on the T-Mobile gateway. That means you'll need to use a web interface to program the extender. The instructions are pretty clear on how to do this, but you might want to review the manual before you pull the trigger.

¡Buena suerte!

My bars vary from poor to good, yet I've never had a TV streaming problem. The picture is ALWAYS clean and crisp even when the bars say poor. I would watch your TV over time, and forget about the bars if you don't see an issue.

@Crison the following thread on the community forum shows that some people have had success using the older Apple AirPort Extreme devices as extenders. Another user uses a EAP 1250 access point. 

Generally, most users have success creating a mesh network at home using a mesh wifi router. My system is Gateway > Netgear MR60 mesh > devices and works great. 

I hope this helps!