i compiled stable 21.02.0 for MR6400v3... you can try this firmware...
i add few line in qmi.sh & qmi.js... Add script for WAN & SIGNAL STRENGTH led
Don't forget to reboot after finish flashing...
your firmware saved my mr6400. cellular is working now.
but i wonder why 23.05 openwrt doesn't work on this device. i installed qmi by the official guide, it is in "connected" state, there is some data in TX but none in RX.