Blokada Cloud with the Apple profile downloaded from app.blokada.org uses DNS over HTTPS.
I believe this is only supported from MacOS 11 and newer.
It’s not possible to simply set an IP in your network settings because Blokada only supports encrypted DNS.
I’d highly recommend you to upgrade your mac since Catalina has reached end of life and might not receive security updates anymore.
If you’re not able to upgrade your mac, there might be 3rd party apps for enabling DoH/DoT on older MacOS versions.
Another solution might be to use Blokada Plus together with WireGuard which will not require DoH support as long as you have the VPN connected.