I had openwrt 21.02.0-rc1 installed on my BT Homehub 5 running without problems. By mistake i installed on LUCI the older firmware 19.07.0.-rc2 and bricked my device.
I connected via serial. This is the log serial log
Your log looks fine, kernel boots up, interfaces are initialized. Have you tried failsafe mode first?
I think you're running into an issue where LUCI/Network config/etc won't work correctly because it's using 21.02 settings and syntax in 19.07. Booting into failsafe mode means you boot the router with a clean, fresh config from ROM. You can then decide to SSH into the device to fix the configuration issues, run the firstboot command to do a full wipe of the old configuration, or try to upload new firmware.
You can also enter failsafe via your serial by interrupting the boot process:
Press the [f] key and hit [enter] to enter failsafe mode
Press the [1], [2], [3] or [4] key and hit [enter] to select the debug level
While on failsafe mode, i run firstboot and it returns:
I root@ (none) :/# firstboot [
24.630996] random: crng init done
I 25.136940] jffs2reset: This will erase all settings and remove any installed
packages. Are you sure? [N/y]
[ 28.991016] jffs2reset: /dev/ubi0 2 is not mounted
[ 28.994607] jffs2reset: /dev/ubi0 2 will be erased on next mount [ 29.000650] jffs2reset: writing /dev/ubi0_2 failed: Operation not permitted
The firstboot command is partially broken on the HomeHub 5; jffs2reset is not suitable for UBI volumes.
One of the following might work - I do not remember which one I used last time I needed this.
run mount_root before firstboot
run mount_root, then erase the contents of /overlay using rm
erase the ubi volume named rootfs_data using ubiupdatevol -t /dev/ubi0_2; but before you do this, you might want to double check the volume name using ubinfo /dev/ubi0_2
perform a firmware installation with sysupgrade -n (do not save configuration over reflash)
Otherwise, as you appear to have serial access, you could boot the 'LEDE installation image' as described in section 4, skip the 'prepare' script and go straight to executing sysupgrade command (try with '-n' flag), to installing 19.07.6 sysupgrade.bin image from USB drive.