FM breakthrough IF is recognized by being untunable, So changing the dial does not alter the received station.
This can happen after you remove shielding cans or break shielding cans solderings.
If it is changing with the dial your oscillator runs at some other frequency.
As this is a cpu pll controlled set it means the pll can not lock.
On the boards a pll chip is located.
Maybe some solder fallen in or a soldering broken in that part ?
If you're not capable of measuring frequencies inside that radio you best find someone who can otherwise use the service manual (
https://www.pe2kmv.nl/wp/en/rx-only-...corp-r-535-uk/ ) to measure and find that fault.
Don't forget it is older equipment now and problems will arise.