So basically you already have 2 routers, neither of which currently supports OpenWRT?
The best advise probably is, to get a supported device
- take a look at the supported hardware list and compare that with what your preferred online shops +what ebay previously owned have to offer. 20-40 bucks and you are in on the reasonably lower end.
- E.g look for MT7621 devices or TPlink Archer, but lots of others are ok too
- the end of thread What's your favourite cheap LEDE/OpenWrt device? is also a good starter