have you been able to solve this?
It looks like the correct firmware might have been:
General_DVR0404LF-AS_3104H_Eng_N_V2.608.0000.7.R.20170312
But if you can't see your device on the network, you probably can't telnet into it.
If you open it up and look carefully at the board, you will see printed on the circuit board RS232
It will have 3 or 4 pins.
Get an RS232-TTL module, find it on ebay.
I think you can transfer new firmware through using a null modem cable. I'm not 100% sure about this.
I don't know the steps, but I'm sure there are guys here that can do a firmware upload this way.
have a look at this guy's video