Can't get WN3100RP to work!

I can NOT for the life of me get this repeater to work and keep working.

The first time setting it up took forever, as the thing kept complaining about it not being close enough to the main router, but it kept doing that even if I'd just push it up against the housing of the router.
At one point I got it set up as a repeater, which, as far as I can remember, helped the WiFi-signal a little bit.
But first of all it wasn't set up to have a password, and either before or after I added a password it just suddenly forgot its settings.

Anyway, I just got frustrated and gave up on it, and over the recent months I've made multiple attempts to get it to work, but just nothing.

All I can do is hope that it will show up on the network, and even if it does and choose it with a mobile device, it goes to the Netgear Genie, asks me to pick my existing network, then enter the password, it exits and the device attempts to join the network after I enter the password in the device-prompt, and then it connects but once again it opens the Netgear Genie and goes through the wizard all over again. - I've been through this loop about 50 times and it's like arguing with a robot.

Occasionally it would tell me there was an issue and tell me to add the MAC-addresses it shows me in case I have an access-list, which I do. But I've added both those addresses and it doesn't seem to matter.

I COULD get access to the repeater itself by using a wired connection, which wasn't recommended in the instructions but I tried it anyway.
However, when I attempted to update the firmware, which took way too long for a non-mechanical device with chips, it suddenly didn't allow me to access it (by using the repeater's IP in a browser) anymore. Somehow updating the firmware seems to break it. - Also, it still said there was a newer version after updating it, which didn't make sense.

Finally, I've also tried the WPS-functionality that both my router and the repeater have, but it just starts searching and they refuse to connect and it just times out. I've tried both with and without using a PIN. - Just nothing, they seem to almost completely ignore each other and refuse to cooperate.

In any case, I can not get this thing to just work. What do I do? Where do I start? I've followed multiple instructions.

What are the things that can prevent connections other than an access-list? - Could this thing be bugged or broken??

If so, could I get a replacement somehow after all these months? I've not even been able to use it.

My main router is a WNR3500L. Which HAS been working well for a few years now. Maybe it's not compatible with the repeater? Which would be weird...
By the way; Can this router be expanded with a USB-antenna or some such thing?...

Please let me know what I can do.