I just replaced my radiator, and the 2 fans in my car won't come on when the engine heats up, only when I put the A/C then the fans comes on. Isn't the fan suppose to come on when the engine heats up? How long does my car have to be on before the engine is heated enough for the fans to come on.

This link should help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-9.html#post1642158

You may also find this link of interest:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235973

The fan on the driver's side is the AC condenser fan, while the fan on the passenger's side is the radiator fan. Ensure the coolant is full and there's no air in the system.

I checked the coolant and it is fine. Everything else is fine. Is just the fan won't come on when the engine heats up, but the gauge inside shows the needle between hot and cold.

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Where does the radiator fan connects to the radiator? What is the part called?

I checked the coolant and it is fine. Everything else is fine. Is just the fan won't come on when the engine heats up, but the gauge inside shows the needle between hot and cold.

Where does the radiator fan connects to the radiator? What is the part called?

It's called a thermo temp sensor. Mounted to the bottom of the rad.
When "it" sees coolant temps well into 200*, it will trigger the fans to come on full speed.
If it stays cool at the bottom of the rad, and the top is very hot, then coolant isn't circulating.
Like a bad thermostat or clogged rad core. Not implying yours is this way.
You sound very normal to me.

Could my problem for the fans not coming on when the car is heated up be the Thermo temp Sensor? My gauge is showing that the car is not over heating. Am just confused with why the fan won't come on. I bought a new thermostat

https://www.askmehelpdesk.com/cars-trucks/when-asking-about-your-vehicle-303903.html
As kitch428 stated, your fans won't come on until the engine reaches a spec temp. They should come on and run if A/C is selected and the vehicle is still for a period: i.e. no air flow across the condenser.
If you tell me what you're driving, I can give you the exact spec temp, if available for your make and model.