The TL-WR840N v5 has too little flash to accomodate current OpenWrt, see Report Devices Here With 18.06.0 Provided Image too big to save overlay
Solution: Get a new device with sufficient flash + RAM.
See also:
https://openwrt.org/supported_devices/432_warning
https://openwrt.org/supported_devices/openwrt_on_432_devices