Fridge not cooling after forced defrost

I have a American-style Samsung fridge/freezer model SR-S2028CVW that has served me very well for many years until recently it starting making a rumbling noise that would stop whenever I opened the fridge door. A bit of Google research told me this was because the fan for the compressor would be rubbing against a build up of ice and a defrost should sort the problem. Even better, there's a hidden service feature whereby if you hold the Quick Freeze and Fridge Temperature buttons down together for 10 seconds, wait for the beep and then press the Fridge Temperature button a couple of times it'll go into a 'forced-defrost' mode and you can get the whole process done in about 20 minutes.

Bingo, I thought. As I have a beer cooler, I can pop the contents of the Samsung in there to save from spoiling and have this all done by lunch.

So I pressed the buttons and everything seemed to be going as expected; the Samsung beeping away for about half and hour then the compressor came back on. But the rumbling noise remained, but as the freezer temperature was still -6c, so I repeated. I repeated 4 times until the freezer temperature was +18c. At that point I thought oh well, just gonna have to live with it.

I let the freezer do it's thing (which it did, dropping back again to -20c), but meanwhile the fridge temperature was flashing '3c'. Bit of Googling suggested there was a problem and switching the whole thing off, leaving it for five minutes and then turning it back on again should solve it.

On turning the unit back on, the fridge temperature did indeed stop flashing '3c', but was now solid at '18c'. I figured it probably just needed to be left alone to get on with cooling down so I left it overnight.

Unfortunately it didn't seem to want to cool. In the morning it was still 18c. I've since tried turning off and on again three more times, but to no avail.

So the freezer side is working fine, but the fridge isn't cooling. The noise remains, but I care less about that than a cool fridge. Given the fridge stopped cooling after the button-pressing, I'm inclined to believe I've put it into some strange mode that it's not telling me about.

I'd really like to get this one back to my pre-button-pressing frenzy just so it can last long enough for lockdown restrictions to be over! Any ideas?