I had changed router and unable to login to my QNAP thereafter.
I had tried soft reset (hold reset button for 3 sec) but unable to login in with default username: admin and default password: admin.
My QNAP model is TS-228A and version is 5.1.9.2954.
If your NAS is registered with QNAP you can log into your QNAP account and get the cloud key for that NAS. Not sure if you can create QNAP account after the fact and get it, but I think you can create an account with the serial number and then log in and get it?