Unable to read sim card

I'm about to by a used phone but (Note 10 + 5G) N976N

But it had a custom N975F firmware on it, in order to convert it to dual sim and a modified IMEI

I wanted to restore it to it's original state so I downloaded the latest original firmware from samfw.com and flashed it

After flashing, the modified IMEI is still there and it doesn't recognise sim cards.

How can I restore the original IMEI to fix the sim card read issue?