Failing to run opkg in dd-wrt on ASUS rt-ac66u

Question

I'm trying to install opkg on my ASUS rt-ac66u router running dd-wrt framework. I followed instructions in this link, but it didn't work!

Keep in mind that I'm installing it on router's internal Flash Storage, not an external USB. But I don't think this should cause any problems.

Problem Defenition

After running

ipkg -d /jffs install http://downloads.openwrt.org/attitude_adjustment/12.09/brcm47xx/generic/packages/opkg_618-3_brcm47xx.ipk

I tried to run opkg but dd-wrt can't seem to run it! it says: -sh: ./opkg: not found

It finds opkg in it's PATH (because autocomplete works correctly), and opkg is present at /jffs/bin/opkg.

Things I've done so far

At first, I suspected that I installed the wrong version (one for a different architecture) because if you run an executable compiled for a different architecture, it shows the exact same error!

But then I looked at this link and found out that the target for my router is ``brcm47xx`, which is the same target that I used.

Then I restarted the router, and still got the same result!

I checked file type using file opkg (ran I my own machine, because dd-wrt has not file command!) and got

opkg: ELF 32-bit LSB executable, MIPS, MIPS32 version 1 (SYSV), dynamically linked, interpreter /lib/ld-uClibc.so.0, corrupted section header size

I ran ipkg update, it ran correctly and tried to update opkg, but at the end I got the same results.

How can I fix this?