FIXED !!!!
You clearly know something the rest of us don't. :)
Loading RUT9XX_R_00.06.08.6 through the bootloader menu worked without the Checksum error.
I've now upgraded the FW to the latest 07.. and all working fine.
Batch 118
Serial 1112669161
From a previous question in this thread
root@Teltonika-RUT950:~# hexdump -C /dev/mtd1
00000000 00 1e 42 4d 90 3c ff ff ff ff ff ff ff ff ff ff |..BM.<..........|
00000010 52 55 54 39 35 30 55 30 32 58 58 58 ff ff ff ff |RUT950U02XXX....|
00000020 30 31 32 33 34 35 36 37 ff ff ff ff ff ff ff ff |01234567........|
00000030 31 31 31 32 36 36 39 31 36 31 ff ff ff ff ff ff |1112669161......|
00000040 30 31 31 38 ff ff ff ff ff ff ff ff ff ff ff ff |0118............|
00000050 31 37 31 34 ff ff ff ff ff ff ff ff ff ff ff ff |1714............|
00000060 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
*
00000080 37 30 31 35 33 36 39 32 ff ff ff ff ff ff ff ff |70153692........|
00000090 73 36 43 31 4c 67 45 62 ff ff ff ff ff ff ff ff |s6C1LgEb........|
000000a0 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
*
00010000