@safaka Even that the model and HW version are correct It looks like you have a customized FW uploaded on the device by your ISP (TurkTelekom). Hence the reason it won't accept the stock FW from...

@paulcanning Click on Edit in Modify column and see if you can change the setting: After the change click on Save button to save the changes.

Just look at bottom of your device - there should be a sticker - check the version. If it says (EU) and HW v3, then you can go ahead with the latest from 2020-03-10.

@safaka If you are with EU firmware then check this link.

Just make some notes on your configuration before resetting, to be easier for you restoring them back.

Try using the C5's blue (WAN) port to connect to your main router/modem device, when C5 is in AP mode and see if that helps.