Bit of forum search might reveal the solution.
Never mind that TOS5.0 is currently based on OpenWrt 19.07.2 and thus there would be no benefit with that upgrade approach.
As far as I know OpenWrt does not fiddle with the u-boot (boot loader) image that ships on the device (/dev/mtd0) from the manufacturer but it does change the boot (env) variables as per OpenWrt specifications [1] and which are different from TOS.
You would have to decide whether to run OpenWrt or TOS and install accordingly but trying to switch between the two on same partition does not work so well.
Alternatively it should be feasible to install TOS on one partition and OpenWrt on another one and switch between the two (dual boot) from ssh. But that is a more complex topic and already discussed elsewhere in the forum.
[1] https://github.com/openwrt/openwrt/pull/2693/commits/bd8b290f4a61f08a9f50a80abae6c8c6070745fb