I don’t see problem with WP76 R16 with mangoh red board, please note that you need to change the state of GPIO2 to initilize the eth0 interface:
root@fx30:~# ifconfig eth0
ifconfig: eth0: error fetching interface information: Device not found
root@fx30:~# echo 2 > /sys/class/gpio/export
root@fx30:~# echo out > /sys/class/gpio/gpio2/direction
root@fx30:~# echo 1 > /sys/class/gpio/gpio2/value
root@fx30:~#
root@fx30:~# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 6A:EF:D0:EE:03:36
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
root@fx30:~# cm info
Device: WP7609
IMEI: 359782080112345
IMEISV: E
FSN: WA803370312345
Firmware Version: SWI9X07Y_02.37.03.00 73df45 jenkins 2020/04/08 10:59:14
Bootloader Version: SWI9X07Y_02.37.03.00 73df45 jenkins 2020/04/08 10:59:14
MCU Version: 002.015
PRI Part Number (PN): 9908102
PRI Revision: 001.000
Carrier PRI Name: GENERIC
Carrier PRI Revision: 002.095_003
SKU: 1103843
Last Reset Cause: Power Down
Resets Count: Expected: 122 Unexpected: 0
root@fx30:~# legato version
19.11.2_4ad011d85a76309438be40fc19e7153a