The D-Link T-series products are based on the Texas Instruments (TI) AR7 communications processor (T = TI, geddit?). The AR7 uses a bootloader / monitor called ADAM2 which is held in flash memory. Unless the area of flash memory holding the ADAM2 code has been corrupted (unlikely) then the router can be recovered by accessing ADAM2 directly.
The usual problem with a 'dead' DSL-G604T is that a firmware upgrade has 'gone wrong'. This can readily (?) be overcome by resetting the environment variables which determine how flash memory is 'partitioned', then loading the firmware and factory default config settings again.
This procedure can be done 'by hand' (which is instructive, but can be technical) or by using a recovery utility and a script to automate things. Unfortunately, D-Link has not released a recovery utility program, but they have released some info which you can find on the D-Link AU site here. Unfortunately, there is a small snag with this information - the default IP address of a UK-spec device is 192.168.1.1 *not* 10.1.1.1 so you will need to do some editing. If you want to follow this route, you can read more about upgrading to AU firmware on the D-Link UK site here.
For more info on ADAM2, start here then follow the links for ADAM2. The issue that you need to be aware of is that V1 firmware uses two separate binary images (kernel and filesystem) loaded into two MTD 'partitions', whereas V2 and V3 firmware uses a single binary image loaded into a different MTD 'partition'...
There are a number of third-party utilities which can help with the process, both from other vendors of TI AR7 based products, and from self-help user groups. Perhaps the best (?) of these is CICLaMaB. Or you could use the TI utility adam2app.exe.
I'll (re)post the necessary MTD settings for both V1 and V2 and later firmware later (when I'm on a different system). It really is quite straightforward when you've done it once or twice...
Hope this helps!
NOTE: Some later D-Link T-series products use an alternative bootloader to ADAM2 called PSPBOOT. I don't have personal experience of a PSPBOOT-based router.