[droid-I]

Seem like it has been confirmed that dd-wrt firmware for
TP-Link TL-WR841ND V8 is compatible with TP-Link MR-3420 v2.4, except no USB support.

Would like to put a request or question whether some dev
could put together a firmware build that would include USB support including mobile broadband?

The limiting factor here is that the above units have only 4MB of flash memory.
Thus only "mini" builds can be used and they do not include USB support.

To include USB support would then mean to leave out some part(s) that are less relevant/useful
as to keep build size below 4MB.

I see there are some guides how to e.g include USB support components etc.
I do not have experience with dd-wrt compiling so I pass this to dev community if someone willing to make this?

The MR-3420 v2 is a quite popular unit and sold worldwide, due to its compatibility
with a large number of mobile broadband 3G/4G dongles.
Many have also put Openwrt on this.

For other routers based on Broadcom I see there are dd-wrt mini builds that have USB support so I guess
this would be possible to do with Atheros also.

For WR841ND V8 there is a factory-to-ddwrt build, so guess same/corresponding is needed for MR-3420 v2 also.

example ref thread here :
http://www.dd-wrt.com/phpBB2/viewtopic.php?t=278951&highlight=

http://www.dd-wrt.com/phpBB2/viewtopic.php?t=165911&start=15

[js1662]

Check the status here: http://www.dd-wrt.com/wiki/index.php/Supported_Devices

Just be patient, even if someone willing to donate an unit, I read post saying that the waiting queue is kind of long.

[droid-I]

I agree that doing this without having a unit to test it on is definitely a limiting factor.

Another approach could be :
The TP-Link TL-WR842ND V2 which is like a "sister unit" to the MR-3420 v2.4,
except it has 8MB flash.
Using the build for tl-wr842nd v2 and carve out some parts to make it fit into 4MB, and see to that the libs for mobile broadband are included?

[droid-I]

bump.
I am not aware how the features are packaged at compile, but two features can be left
out of a potential DD-WRT MR-3420 v2 build, to likely make it fit into a 4MB flash :
IPv6 and PPPoE.
Ie a "mini" build made out of TL-WR842ND V2 dd-wrt.
The Openwrt/LEDE custom made Rooter builds with 3/4G USB dongle support
seem be some 3.6MB.

For the newer MR-3420 V3 (4MB flash) Qualcomm
QCA9531 @650 , compatible unit seem be WR842N(EU)V3, 16MB flash.