When the tax is paid as IGST whether the PoS has been mentioned wrongly. So tax must have gone to the wrong State.
Irrespective of this, the fact that the rectification/revision of this mistake is now not possible in the hands of the return filer, ideally the taxpayer should not be put into trouble.
You should take the notice and fight wherein you can take a plea that the department should rectify the mistake in GSTR-1 and the appropriate State gets the tax. This will be a very long process. Maybe tribunal (if it is formed and the case gets listed) could give relief, else only at the High court level.
Else you could pay the tax and go for refund of the IGST already paid. Ensure your documentation is in place and once you pay the right tax with proper POS your time limit for 2 years for claiming refund, will start therefrom.