As you have replaced RouterBOOT with u-boot. Reverting this is a bit cumbersome and dangerous if you do not have a SPI programmer.
You need to write RouterBOOT+RouterOS or RouterBOOT+OpenWrt(new) to the flash.
If you have made a backup of RouterBOOT+RouterOS before installing u-boot and OpenWrt(old), you should use your backup. Or you may use the backup provided by me:
Please use at your own risk. Flashing my provided RouterBOOT+OpenWrt (snapshot) may be simpler because you do not need to install Openwrt again after that.
To flash the full flash backup (16 MiB), the safest way is using a SPI programmer, because with a programmer you can recover possible bricked devices.
Without a SPI programmer, you need to flash the backup using the u-boot command line or an Openwrt(old) build that unlocks the u-boot, the u-boot-env, and the factory partitions. I'm not familiar with these procedures but I know they are dangerous.
As a workaround, you can build your OpenWrt for that device, reverting the changes to the partition and image generation. It is safer but you need to build OpenWrt every release on your own.