RBR50 permanently pulsing white

Hi all,

My RBR50's internet connection dropped out yesterday for no apparent reason. I powered it off and on again and it now refuses to boot. I initially get a fast flashing white ring, with a green power LED. After a minute or two it goes to a slow pulsing white and sits there without doing anything further.

I did not upgrade my firmware recently, nor were there any power problems or storms. I was already runing 2.7.3.22.

I've read a number of posts on here before posting myself. I've tried:

1. Holding down the factory reset pin. The power light changes to amber, then flashes amber. I then reboot the device and the same thing happens.
2. Holding down the factory reset pin until it flashes, power off the modem for 30 seconds while still holding the pin, then starting it up again while still holding the pin. I get the rapid flashing white ring initially. The power LED then goes solid red, then starts to blink red. Once I let go the power LED goes green, the ring goes blank and nothing further happens.
3. I've attempted to TFTP a firmware file. After following the steps in my first dot point I cannot establish a connection to upload the firmware.
4. I then powered the device off, held down the reset pin and then powered it back on. Once the power LED flashes red, if I continue to hold down the pin I can establish a TFTP connection. I get a successful transfer message, however once I release the reset pin the ring and power LED both go dark and nothing further happens. If I power the device off and back on, I get back into the same loop I started with.

The device is three years old so I'm outside the RMA/warranty period. Before I throw this thing out, can anyone think of anything else I might try? Uploading the firmware file while holding the reset PIN isn't something I found anywhere on the forums, it's just something I tried out of frustration. It appeared to work to a degree, but I'm not sure what state the operating system is in that it accepts the file transfer. So I'm not sure whether that's actually a step forward or not.

Thanks in advance for any ideas.