IPhone is supposedly connected to Wifi but has no connection to neither internet, nor local network

It has been two days of google-fu and I'm really out of ideas here. I live in Germany, just in case that's important.

Current situation

For a week now I haven't been able to connect my iPhone 5 (iOS 9.3.1) to the internet over my home Wifi. My router is a TP Link Archer VR200v (Firmware version 0.8.0 0.21 v002f.0 Build 151105 Rel.35502n, it's the latest one.), I think the international name of it is AC750. I use the VR200v as a router and modem.

Our DSL was down for a few minutes last weekend. While I'm not 100% sure those two things are related, ever since then, the iPhone hasn't been able to properly connect to my Wifi. The phone claims that it's connected but I can't open any websites and also can't access my router's configuration interface (192.168.1.1). I don't have any problems connecting to other Wifi networks and using their internet, I tried my university's Wifi and also used my other phone as a hotspot (Moto X Pure) which both work flawlessly.

I also don't have any problems connecting other WLAN devices to the network. Two laptops, TV, the Motorola, all work and have stable internet connections.

What I've tried

The router allows 2.4 and 5GHz frequencies. I tried both, with and without WPA2 passwords. Same results.

I wiped the iPhone and restored it from a backup, hoping to clear some sort of messed up configuration. It didn't make a difference. I tried connecting both the 2.4GHz and the 5GHz Wifi during the setup without restoring it first, same result.

I tried a different Firmware for the router, still no connection.

I tried manually setting the channel (tried several for 2.4 and 5GHz) and the channel width, which had been suggested by people with similar problems, that didn't help though.

Other people suggested disabling WMM on the router (i.e. http://www.ceus-now.com/wifi-connection-problem-iphone-to-tp-link-router/)

The problem is, I haven't found a way to disable WMM on my router. It claims that WMM is obligatory for the 802.11n und 802.11ac modes, which suggests that 802.11g allows disabling it. However I can only choose between 'Only 802.11n', '802.11gn mixed' and '802.11bgn mixed'.

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So I'm really out of ideas here. I feel like some setting hasn't been deleted 100% and creates some sort of issue between the phone and the router. Has anyone experienced something like this before? I'm open for suggestions!