Hi there,
I tried to install (upload) the "openwrt-18.06.2-brcm63xx-smp-DGND3700v1-squashfs-factory.chk" to
my Netgear DGND3800B using the Netgear Web UI Firmware Upgrade option.....
unfortunately it did not work..... no valid Firmware File recognized message appears..
is there a different way to get openWrt installed on my Netgear?
https://openwrt.org/toh/netgear/dgnd3700
Best Regards
d u t
Well, it appears to me that the page you mentioned has different build for your DGND3800B
http://downloads.openwrt.org/releases/18.06.2/targets/brcm63xx/smp/openwrt-18.06.2-brcm63xx-smp-DGND3800B-squashfs-factory.chk
still the same message:
after trying to upload message:
*Firmware upgrade failed. Please check your file.
appears 
In the same page you mentioned, at the bottom, there are instructions to flash using TTL + TFTP.
That involves opening up the router and using a specialized TTL adapter ($3 ~ 5). Some resources would mention soldering, but often you can do without.
I hoped I could take the good weather method to get openWrt installed 
but yes I just soldered in a pin header, where RXD, TXD and GND for the TTL-com are located
.... I already ordered some TTL-UART---USB adaptors 2 weeks ago (for different purpose) but did not arrive yet.......
but at least.... when it arrive... "now" it's just plug and play (using jumper wires)
Best Regards and thx
d u t
I could set up a raspberrypi to do the TTL-UART job.... let's see what is going to win: lazyness or impatience...
I'm sure you would know that already; the Rx and Tx on the pictures for the router board would be those of the router's. So you would need to connect Rx to Tx and Tx to Rx.
Of course..... General thing in communication....
A mouth (tx) belongs to an ear (rx).....