As FRITZ!WLAN Repeater 450E is supported, maybe porting OpenWrt to this Router is possible?
https://boxmatrix.info/wiki/FRITZ!Box_6820_LTE_v2
LTE m2 Card: L830-EB w/ Intel XMM7262
http://www.fibocom.com/prod_view.aspx?TypeId=67&Id=308&FId=t3:67:3
Thanks 
Necrobump!
Porting should be possible, but unfortunately I don't know how. I would be willing to test however. The Fibocom 4G card should be supported in USB mode, which the v2 fortunately uses.
usb 1-1: New USB device found, idVendor=2cb7, idProduct=000b
AVM introduced a bug for me that makes this router annoying to use when it disconnects from the internet, and hasn't fixed this after months.
Stock partitions:
[ 0.363188] Creating 3 MTD partitions on "ath-nor":
[ 0.368131] 0x000000000000-0x000000020000 : "urlader"
[ 0.374143] 0x000000020000-0x000000090000 : "tffs (1)"
[ 0.380223] 0x000000090000-0x000000100000 : "tffs (2)"
[ 0.386305] Atheros on-chip NAND FLash Controller Driver, Version 1.0 (c) 2014 AVM GmbH, 2010 Atheros Communications, Ltd.
[ 0.397949] ath Nand ID: 98:f1:80:15:f2
[ 0.401775] Toshiba TC58NVG0S3HTA00 1G 3.3V 8-bit [128MB]
[ 0.407245] 5 avm_nand partitions found on MTD device ath-avmnand
[ 0.413382] Creating 5 MTD partitions on "ath-avmnand":
[ 0.418668] 0x000000000000-0x000000400000 : "reserved-kernel"
[ 0.433177] 0x000000400000-0x000003400000 : "reserved-filesystem"
[ 0.532382] 0x000003400000-0x000003800000 : "kernel"
[ 0.546006] 0x000003800000-0x000006800000 : "filesystem"
[ 0.649919] [rootfs] use mtd6 (filesystem) as root
[ 0.654749] 0x000006800000-0x000006a00000 : "config"