Hello, I've installed openwrt on this router without knowing that wifi is disabled on first run.
My router has a dead ethernet port and now I can't access it in any way.
I also don't have a serial cable and I've searched everywhere for one but couldn't find any 
Is there another way to use the ethernet port on it? Or maybe another solution to bring it back to life?
Well, if it's physically damaged then installing OpenWrt (or any other firmware for that matter) wouldn't fix it.
You could get a cheap one from ebay for a pound or two, while a genuine one would cost about 5. Not that you would need a TTL cable not an RSS 232 cable.
Alternatively if you have a Raspberry Pi (or similar, with a header) you can use it for that purpose.
But then again, how good an access point without an Ethernet port (if it's indeed damaged). Yes, you might be able to use it as a wireless bridge or so, but then if and when you have a problem you will need serial access, so not very convenient, especially that it's a cheap device and, probably, not worth the hassle unless you are doing it for fun.
I know it's not going to fix it 
Basically I just want to turn on wifi again so I can try openwrt and see if it's worth it
Unfortunately I live in a place where there's no ebay or Amazon or any other online service like that
I think it's probably useless right now and fixing it is not worth it's value (I actually bought it for a dollar so even buying a serial cable is not even worth it)
I just wanted to try openwrt because I never tried it so that's why I bought it but I didn't know about the wifi thing.
At this point, without functional ethernet, there is no way to access the device without serial console access - and going out buying one (and soldering supplies) to recover this specific device is hardly worth it.