Hello. My WiFi is not working on my iPhone

Are you asking about Wi-Fi or mobile (cellular) data? An “unlimited plan” usually refers to mobile or cellular data, not to Wi-Fi. Except in very, very rare situations Wi-Fi is always unlimited.

So, specifically about Wi-Fi:

• You need a Wi-Fi router or hotspot near your iPhone (near is about 150 feet)
• Go to Settings/Wi-Fi; there should be a switch If the switch cannot be turned on your phone probably has a hardware problem, and if you go to Settings/Bluetooth and you can’t turn it on either it is definitely a hardware problem. For a phone that old it is not worth the investment to get it fixed; consider a new phone.
• If you CAN turn on the switch, tap on a known Wi-Fi network and enter its password when prompted - If you can’t connect follow these instructions→If your iPhone won’t connect to a Wi-Fi network 

For Cellular or Mobile data:

• Do you see signal strength bars, or does your phone say SOS or No Service; for either of these see→If you see No Service, Searching, or SOS on your iPhone or iPad
• If you see signal strength bars go to Settings/Cellular (Settings/Mobile Data for non-English language phones) and make sure it is turned on and enabled
• If neither of these solve the problem, contact your carrier

One more thing:

If you are using VPN, DON’T! Delete any VPN apps and profiles.

Just to double-check are you referring to Wi-Fi or cellular Internet access for your iPhone? AFAIK, there are "unlimited plans" for cellular, not Wi-Fi?

Regardless, if you are referring to Wi-Fi, is this phone not able to connect to only a specific Wi-Fi, or any Wi-Fi?

dori ~ Does your iPhone 6s Plus still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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