I established a TFTP server on 192.168.1.10 (Laptop LAN)
I put in the initramfs image, called it ap.bin
Connected router's WAN port to my laptop LAN
Held the WPS button while booting router. I picked the image successfully
I was able to ping the router, but could not access it via web UI. I could ssh into it as root and login.
At that point I was confused as to what is the next stage so I decided to take a guess - so I downloaded the http://downloads.openwrt.org/releases/19.07.3/targets/ath79/generic/openwrt-19.07.3-ath79-generic-pcs_cr5000-squashfs-sysupgrade.bin, put it into the TFTP server, renaming it as ap.bin, then repeating the process to get the router to pick it. It did, but instead changed it's IP to 192.168.99.1, with an SSID that I could connect to, but this then made the router give me a minimal interface to login to where I could not change anything.
I believe that the people who developed the firmware for this router meant well. We thank them. However, our understanding of the instructions could be stopping us from succeeding in running OpenWRT on this router.
Have both the initramfs and the sysupgrade builds of OpenWrt downloaded to your PC.
TFTP boot the initramfs OpenWrt again like you did the first time.
Use scp to push the sysupgrade image to the RAM disk of OpenWrt running in RAM: scp openwrt19.07.3etcetc-sysupgrade.bin root@192.168.1.1:/tmp
then continue where @slh said log in with ssh and run sysupgrade. This will reboot and you should then have a permanent installation of OpenWrt in flash ready to configure and use.
I wanted to install a VN client that supports OpenVPN/PPTP/L2TP and so found softether vpn client, which matches exactly what I needed.
However, I am unable to install it because of space limitations on this router. On checking, I see as tmpfs has been allocated all the space that I'd rather have allocated to /, which is where packages are supposed to be installed (/overlay) - at least according to opkg configuration.
Looking at the output of df -h it looks like there is something that is not normal with having 30MB allocated to /tmp and 2MB allocated to /
No.
Tmp is likely on ramdisk, so it is temporary, not flash.
You only have 8 MB flash, based on wiki.
That gets consumed by kernel and roots (rom), and the remaining space is readwrite overlayfs, visible as free space on /
29.1 is approximately half the size of the RAM (after subtracting about 4 MB that the kernel occupies). This is the default limit on the RAM disk. As others said, /tmp is not suitable for installing software because it is erased and repopulated on every reboot. Also storing large files in /tmp takes away RAM that could be used by applications.
I don't know about softether but I have run VPNs OpenVPN and Zerotier on 8/64 hardware. Not at the same time of course. Also Wireguard is reportedly designed to be light on memory use, but I don't have much experience with it.