IPhone 5S not connecting to Wi-Fi?

I need some help. I have a not-too-new TP-link TD-8901G modem. Its been working fine for years. But it has an issue with idevices. Every iPhone i ever tried to connect to this modem, didn't connect. At least for a while. A 4s, a 5c and my own 5s. When i select my Wi-Fi, and enter password it attempts to connect but after a second without any messages it disconnects. It did connect like 2 months ago luckily. But i had to reset my modem because of network problems. Now it doesn't connect again. I tried changing every connection and authentication mode with no luck. Even tried setting static ip which was the reason it worked the last time. But it doesn't work anymore. The problem seems to be happening only between idevices and this modem. Because my phone can connect to any other Wi-Fi and any other device (like my windows laptop or my parents' android phones) can connect to this WiFifin. Can anyone give me some advice?

Give us some more information.

Maybe its your auto correction. Say like your pin is all lower-case. All the Iphones have auto-correction turned on that affects the pin when you entered in. Say like (ex.) the pin is "notyours" and when you type in your iPhone, the iphone turns your pin into this pin instead "NotYours", causing it not to connect because the pin does not match. Pin needs to be "notyours" instead of "NotYours". Change your pin (On your router) into cap then lower-case for the rest. Then try to see if that worked.