Hap ac2 only 128MB RAM

i have a hap ac2 with 256mb ram(when it was on routeros, showed 256mb ram).
On openwrt only 128mb
[ 0.000000] Memory: 116340K/131072K available

How can i make the openwrt show the full256mb ram ?

According to https://mikrotik.com/product/hap_ac2#fndtn-specifications it has 128 MB of RAM, or did you add more memory yourself? Check /proc/meminfo for full details.

As you can see

Early revisions seemed to ship with 256 MiB of RAM (see wikidevi). However, revisions available in 2019 only ship with 128 MiB of RAM.

I have an early revision of the device

Ah, I see, then I'm pretty sure you would need to change https://github.com/openwrt/openwrt/blob/main/target/linux/ipq40xx/files-6.6/arch/arm/boot/dts/qcom/qcom-ipq4018-hap-ac2.dts#L16 to reg = <0x80000000 0x10000000>; and compile it from source since this early variant isn't part of the build system.