Hi,
That is exactly the method I have been using. BOOTP-flashing with TFTP. I also have a serial port connections to the device, so I can see the output:
SERIAL-PORT OUTPUT from BOOT-P attempt
Technicolor Gateway
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Gateway initialization sequence started
Boot Loader Version : 15.38.724-0000000-20150917132051-
CPU : BCM63137B0
RAM : 256MB
Flash : 125MB NAND
Board Mnemonic : VANT-W
Market ID : FFFC
Entering BOOT-P mode (reason: BUTTON_PUSH )
BOOTP Reply received
Local IP: 192.168.10.2
BOOTP Server IP: 192.168.10.5
TFTP Server IP: 192.168.10.5
Filename: 172339w1021008closed.rbi
TFTP started
*** 0 kB received ***
*** 50 kB received ***
.
.
*** 23635 kB received ***
TFTP finished
File is not a valid BLI
Resetting the gateway
I have successfully used this method on a similar Telia (tg799vac vant-r) device. That one accepted the firmware sent with TFTP. I could then do the banck switch with "boot failure" procedure. After that I could use a method to get root.
So my guess is that if I have an rbi-file with old Telenor firmware it might work... 