How to track a lost iPhone when Find My shows no location?

Yes, it's probably lost now for good. If it cannot connect to a cellular network or wi-fi it will be unable to relay its location. But also, as long as it logged in with your account it cannot be used.

As long as you any type of passcode on it, your data is safe and protected.

The only thing you can do now, is keep it in lost mode and on your account. Do not remove it from your account as that would grant full access to whoever has it. Though it won't grant access to your data.

Possibly, but that is unlikely to limit cellular connection to such a degree as to render it unfindable. Probably would not be connected to Wi-Fi but cellular would still work to some degree, unless its like in a basement somewhere 10 feet underground where cell signal cannot reach it.

If left in a hotel room drawer or even the manager's office it would likely still get cell signal to some degree.

The other possibility is, it's simply dead now, out of battery. It's gone 10 days without being charged so that's also a real possibility now.

The fact it's not findable usually points to someone actively disabling the option by removing the SIM card. But even if not the case, then it's just as likely to be out of battery at this point.

But either way, you are unlikely to ever find it again.

Re: “… but Its definitely not stolen if I'm right …”

Not really.

A thief will take active measures to prevent a phone from being tracked w/ FindMy; giving the same indications observed by the OP.

It’s covered in some detail in “Phone Theft 201”