old 4S to connect to Wifi, but it still says "Unable to Join the network".

@271apple: You are doing a great job leading this complex troubleshooting exercise! I have just a couple of additional suggestions:

• It wasn’t clear if @jro.alan actually shut down the router completely and restarted it. This is an important step, because routers are computers also, and can get flakey if they aren’t periodically restarted, just like any other computer.
• Check the router settings for MAC Filtering (aka Access Control) - if it is enabled, the Wi-Fi address of every device that “wants” to connect to the router must be on a “whitelist” in the router settings. The easiest way to check this is to turn off MAC filtering if it is on (MAC is not an Apple computer; it is “Media Access Control”).
• VPN: If the 4S is using VPN that can prevent the device from connecting - delete any VPN apps and their associated profiles
• It is NOT a router password problem - the router would say “incorrect password” if that was the problem

jro.alan ~ Wikipedia says: "The 4S was officially discontinued on September 9, 2014 (...) although production did continue for developing markets until February 17, 2016."

...So that original battery is at least 6 years old, likely to be weak and so unable to connect to Wi-Fi. If you can find somebody that would still have a new battery for such an old model, have it replaced and all will (probably) be well:

iPhone Battery & Power Repair

About genuine iPhone batteries

Repair Provider Verification

(Articles found by searching Apple's Knowledge Base.)

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Have you tried turning off MAC filtering? Or adding the phone’s Wi-Fi address to it manually?

Have you tried restarting your router (even if other devices connect OK)?

I'll get back to you as soon as I can. I'll try to find some answers. I'm sorry that none of my answers worked... I tried everything I could and I can see why its your last Apple product haha. There's one more: Apple support. You can call +1 (800) MY-APPLE or by going to support > iPhone and scroll until you see chat with a specialist. Or search that up. If they can't help, it might be the iPhone. Let me Know if you figured it out so i will know how if other people have this problem.

Sorry about the long reply. I've exceeded the amount I've posted I'm supposed to go, so I had to wait a few minutes. Make sure that you don't have Caps lock on; make sure you have the Right lowercase And uppercase letters; You're doing the right password for the right Wi-Fi network. If these three don't work, let me know and I will go onto the next part of the list.

because "find my" is on.

See the following Apple articles:

Activation Lock for iPhone, iPad, and iPod touch

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

How to remove Activation Lock

How to remove Activation Lock – Apple Support (UK)

• If you need help removing Activation Lock and have proof of purchase documentation, you can start an Activation Lock support request.

You can also contact Apple support for help, -> Get Support (apple.com)

You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.

If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.

Once Apple receive your proof of ownership, Apple will further review your request and provide an update.

because I can't verify my apple ID on the phone

You are sign in here .  .  .  with your Apple ID & Password.

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

What happens when you use the same Apple ID as you use to sign in here?

because it won't connect to the wifi.

Check the Wi-Fi.

I know this one is kind of Stupid but I made it up last minute. Try connecting (By USB) Your iPhone to your router. I don't even know if this will work, but I think it's worth a try. If that doesn't work, Tell me. I can't tell you or figure this out because my Router is being fixed these couple of days, and I'm using my hotspot. Also if android phones have a hotspot, try connecting to that.

It just won't connect to wifi. It won't get a cell signal because I don't plan on connecting it to my carrier.

You need to contact to your carrier / provider to get signal and/or Wi-Fi.

No carrier or no provider is the cause of the problem, that you get the message -> Unable to Join the network

So look for / visit a carrier, take a subscription, postpaid or prepaid card.

Checks whether the SIM card is suitable for your iPhone.

If you have a car, the car needs gas.

The car cannot run without gas.

See this Apple article: If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

See the following Apple articles:

Connect to Wi-Fi on your iPhone, iPad, or iPod touch

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support

Recommended settings for Wi-Fi routers and access points

Recommended settings for Wi-Fi routers and access points - Apple Support

If you need help with your Wi‑Fi password

If you need help with your Wi‑Fi password - Apple Support

If they are On, turn Off Hotspot & VPN.

Connect to any computer USB port, go to settings > reset > erase all content and settings. You don't need find my off. And I know that I'm on level one, but still try it. If it doesn't work, let me know.

Hmm... Could you try sending me a picture of what the 4s says? If you can't that's fine. Just tell me what it says on the screen. And if you could let me know if it doesn't work, that would be really helpful to me.