What should I do so that my iphone connects to known Wi-Fi automatically?

So recently I reset my network settings as someone said that I should do that to lose a problem (my phone was giving me two text alerts when I've only received one. Yes I checked the notifier already which I put to never) so that problem is solved but now everytime I get home, I have to key back in my Wi-Fi password whereas last time it would automatically connect. I've been resetting my network settings a few times but nothing has changed.

Although iPhone does have an option to connect to known Wi-Fi automatically so you don't need to do anything, you should go to Settings > General > Reset > Reset Network Settings.

Then try keying in your Wi-Fi password and next time it should do so automatically.

I would recommened a full back up and update the software or restore as a new phone if its happening with all Wi-Fi routers if it happens only at home you may contact your Internet provider
For more info please check my answer here