Here's how. https://www.google.com/search?q=iphone+forget+network
What I do is forget all networks and then connect to my WiFi at home.
There are similar articles for Android but let's work one phone at a time.
Our old and cheap phones have been working well for as long as we've had them. We just connected them to our host's wifi two days ago, gone in and out of range and signed in and out of other networks and have had no issues signing back in/automatically connecting to our host's wifi. Now, today, we connected both phones to a shop's wifi, did what we needed, and then left. Upon returning to host's: the iPhone 4 can sign into the host wifi, but not reach the internet, and the J3 cannot see our host wifi at all (though can see the neighbors'). Here's the big twist: my laptop, which did leave the house with me but did not connect to an outside wifi - connects to host wifi and internet fine. My partner's Samsung Galaxy S5 (not currently on any network), which stayed at home, connects fine. When I go out, my J3 can see and connect to other wifi networks fine, though it no longer notifies me when a new network is in range. Our theory: somehow the shop's network "messed up" our devices, and now we can't connect to the host wifi/see it at all on the networks list. What the heck is happening, and how do we right it? Or did some bad malware shiz go down when we connected to the shop wifi? Thanks in advance.... - stumped and worried in Dublin