I found a old D-Link DIR-600 on my work with no task....i have at home a Netgear N600/WNDR3700v5 for a WLAN Network Bridge in use for one litte Printer on it .. without WLAN Interface
So i get the DIR-600 and look to take over the Task...connecting a power supply oh booting up...with Firmware 2.12....navie a was... to install OpenWRT 19.07.10 via Web Console...what fails....
So deepdive in the Web for this Device...and the idea in Head..it must work.
First: Found this https://diy.viktak.com/2016/02/installing-openwrt-on-d-link-dir-600-router.html Install Help..ok ok a Tutorial from 2016 what can it helps 9 Years later? Yes it can.
Ok Downloading the modified version of the original firmware via DropBox works Fine...
Then: Putting a fixed IP in my Notebook .. 192.168.0.2
I have connected the DIR-600 to my local LAN...via the WAN Interface - also we have Internet on the Router.
Then we Following the Instructions form Viktor...we should Upload the openwrt-15.05-ramips-rt305x-dir-300-b7-initramfs-uImage.bin on the Router .. but i am a little bit lazy...i do not have a own Webserver ... what we use? Why not the archive from OpenWrt.... go to https://archive.openwrt.org/ and navigate to chaos_calmer 15.05 ramips rt305x and found openwrt-15.05-ramips-rt305x-dir-300-b7-initramfs-uImage.bin and copy the download link - but here is https include...so kill the s form https 
Booting up the Router emergency firmware upload ... pressing the Reset Button simultaneously put in the Powersupply in...so the Router comes Up and present the Uploadpage (192.168.0.1) for the modified Firmware...so now we can Flash the modified Firmware in the first step...
The boot look normal...you can login via Web..or...we use...telnet on port 2323. So i open my Console and we use... telnet 192.168.0.1 2323
Following the Instructions form Viktor...we should Upload the openwrt-15.05-ramips-rt305x-dir-300-b7-initramfs-uImage.bin on the Router.. we use an other way... go the the cd /var and use wget http://archive.openwrt.org/chaos_calmer/15.05/ramips/rt305x/openwrt-15.05-ramips-rt305x-dir-300-b7-initramfs-uImage.bin For the lazy .. copy the link and paste
Now we can flash the Image on the Router with busybox flashcp openwrt-15.05-ramips-rt305x-dir-300-b7-initramfs-uImage.bin /dev/mnt/2 when finished we reboot -f
Ok the first is done....now...we can flash to the last OpenWRT Version 19.07.10 download it form the https://archive.openwrt.org/releases/19.07.10/targets/ramips/rt305x/openwrt-19.07.10-ramips-rt305x-dir-610-a1-squashfs-sysupgrade.bin
Upload via the dialog..wait for the reboot.
Now we a have Router with the last working version of openwrt...
But we are not finished at the step. Now install after Upadte the lists luci-proto-relay... it dependens on relayd...after this we reboot the Router. We can disconnect the Router from the wan port
me helps this tut:
https://www.youtube.com/watch?v=VB7qXWVilws (german)
Notice: do not try to Update the DIR-600 it fails with: no space left on device... 
Hope it helps for anybody