I just can't seem to connect my Roku 3 to the Internet on my new TV, I got a NetGear WiFi extender...etc..

well you have no ISP internet at all;
so only path is CELL
so in the CELL setting, enable, tethering,
4k netflix will fail
ROKU is best in the world, I have 3 hear, and player work now for 8 years no probs.
ever.
but tethering will cause you huge data overage bills
call your Cell phone provider and have this discussion.
or get real internet. (no data caps)
I too am cable TV free, for huge long time, but have internet fast.
good luck to you. !!!!

By connecting to your existing WiFi network and enhancing coverage, the NETGEAR range extender enhances your home WiFi network. The steps below will assist you in setting up your extender.
We recommend using the Nighthawk app to set up your extension on our latest devices. See the following articles for more information:
https://pnews.org/boost-your-speed-with-netgear-nighthawk-r7000/

1. Plugin your extender in electrical outlet.
2. Place your extender near your router when first time your are setting new extender.
3. Launch any web browser and enter mywifiext.net. The New Extender Setup page displays.
4. In the Password section, type the existing WiFi network password. Default user name is admin and password is password.
   

5. Click continue and a message box displays saying that your extender is connected to the new extender WiFi network
6. Now, you can connect your device to the extender network, go to the WIFI settings and connect to the extender's new network.
7. Netgear EX6100 Setup is successfully completed now place the extender at the desired location and enjoy seamless Connectivity.

What router and where is it that you are connecting the Extender to? An extender is not a router. You need a router to extend the wifi signal from.

Have you first established the connection between the Router and the Extender?
If so, have you connected e.g. a laptop to the Extender?
So if your WLAN networks SSID is NETGEAR, the WLAN network of the Extender is NETGEAR_EXT and you have to connect the laptop to NETGEAR_EXT.
Then you have to open your favorit internet browser and have to type in www.mywifiext.net, which I guess is a buildin Setup-Assistant.If this doesn't work, make sure, your laptop is configured, to get an ip address automatically via DHCP.
Or read http://kb.netgear.com/app/answers/detail/a_id/20912