GL-MT300N-V2 Network mode - Extender configuration

I’m struggling to correctly configure the GL-MT300N-V2 to work in Extender network mode.

Scenario

I have several raspberry pi projects on the go and to configure them (headless) I need to ssh in.
In my office i don’t have an ethernet connection but i want a way to plug my Pi into a device and have that connect wirelessly to my network. I haven’t had much luck with wireless dongles i therefore got the GL-MT300N-V2.

My problem.
I thought the correct configuration of the GL-MT300N-V2 would be to use Extender network mode (ui v 3.012). With this setting i can’t seem to access the pi or the router from my network.

I have tried opening ports (80 & 22) on the router but i’m not convinced it is working. I have read that i may have to do this in Luci but that looks above my skill level.

Is the configuration i’m striving for possible with the GL-MT300N-V2?

Can anyone help me solve the issue?
Thanks in advance.

So you want to connect Pi to MT300N-V2 using cable then connect MT300N-v2 to your wifi network?

If you want to set up extender mode, go to more settings → network mode to set up extender. In Internet settings it is WISP repeater which the router works as a firewall.

Another question: why not just use a cheap Wifi Dongle and plug in your PI?