2018 Sonata, Key Fob replacement
 

Daughter lost one of the key fobs. Called the Stealership and was told it'd cost $550 for a new one!!
I see used key fobs online for under $50 and there are plenty of locksmiths who can program. So I bought one. It looks the same on the outside but inside looks different and the model # reads 95440-D3100 which seems to be for Sante Fe. Would this work on Sonata or does it have to be model specific? Upon research, the correct model # for sonata key fob is 95440-C1500NNA.

What if you took it to the locksmith and have him try to program it. If it works , all is well if not ,maybe he can better answer what could be the problem.

The Sonata has a trunk, where as the Santa Fe has a hatch. I am not sure if this different key fob model will actually work? Try to purchase the correct smart key fob yourself, and then take it to a professional locksmith. This way it will be much cheaper than using the stealership.

You can't program it on your own, you need to take help from locksmith experts to program it.

One of the reasons we recommend a certified locksmith is they need to have special clearance to access certain anti theft (immobilizer) info from the manufacturer's website.
Different vehicles have different requirements to program keys and/or key FOBs. Special equipment is necessary and certain conditions must be met to program addl FOBs
Some can be easily programmed by the vehicle owner providing certain conditions are met. What does your owner's manual say in this regard?

Yes, they have special clearance to access anti theft information from the manufacturer website, this is the reason i also recommend locksmith dc for key fob replacement.