how i can undo remove device from find my iphone?

Unless you erased it before you removed it from iCloud, the device will still be Activation Locked. So, if and when it does come back online, I would recommend that you don't erase it and then remove it from your account. That would be like handing the phone to whomever has it and giving them a free phone. You can erase it and leave it on your Find My iPhone account, and the Activation Lock will still be in place. You won't be able to track it, but it will still be unusable for anyone who tries to restore it:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Best of luck,

GB

You will have to wait until the device comes back online. You were only able to remove it because it was offline. Since the device is signed into iCloud with Find My iPhone turned on, it will reconnect to your iCloud account if and when it comes back online, at which point you can then trigger the Erase option.

Best of luck,

GB

No. You can't undo that. You've removed it. The activation lock has been disabled. You need to pay attention to what you're clicking on. It asked you for confirmation when you clicked on remove. There's nothing that can be done about it now.

Can you be more specific? what do you mean

Yes.

GB

It will reconnect to the iCloud account once it comes back online, and if someone tries to restore it while it is offline, the iCloud account will still be signed in and the device will not be able to be restored without the iCloud ID and password that the device is Activation Locked to. The order that the Erase and Remove steps is what is critical here, I believe.

GB

You can do nothing until the phone is back online (if it ever comes back online). Once it does, you can put it in Lost Mode or Erase it as you wish.

Cheers,

GB