I had a D-Link WBR-2310 (2.4 GHz only) setup with WEP. Mine and my wife's iPhone 6 had horrible download and upload speeds. I did a network settings reset on both of our phones along with changing from WEP to WPA2. That seemed to fix the issue, but only for about 12 hours. After ~12 hours, we were having the same issues again. Sometimes, after returning from work or going out and running errands, our iPhone's wouldn't connect back to our home wifi unless we rebooted our phones. I even tested just rebooting the router and not our iPhone's, but our iPhone's would still not connect. We would have to reboot our phones.
So, I decided to get the Belkin N600 DB (2.4 & 5.0 GHz) yesterday. I set up the router with WPA2 and got our iPhone's connected to it. Everything was going good for ~12 hours. Then, the same exact issues we were having with the D-Link WBR-2310 were happening with the new Belkin N600 DB.
Every time this happens, I go into the Wifi settings on our iPhones and there are ZERO SSID's available. I put the phones into airplane mode, then out of airplane mode and still there are ZERO SSID's available.
Only after you reboot the phones will SSID's appear in the Wifi settings again. Then, it will automatically connect to wifi because it finds our how wifi network and connects to it.
This is indeed an issue with iOS 8 and not the iPhone 6/6+ hardware because this same exact issue is happening with our iPad 3 with iOS 8 on it.