Thanks for the tip @RadioOperator!
I just don't want to throw it out in the bin to go and buy another one that is easier to hack...
For the time being, I have a couple of Maconix memories laying around and I am waiting for the socket to arrive. It will be a while...
For those interested, the memory datasheet can be found in:
While things are being shipped I will look into the UART interface and see what can I find. It is quite an old router. I doubt it will be very much locked or protected...