The main issue was the fact the node was outdated. You needed 2.7.1.
The fact that dig @8.8.8.8 _algobootstrap._tcp.testnet.algorand.network srv does not work means you cannot use 8.8.8.8 (and most likely you cannot use DNSSec which reduces security).
But if the correct flag disabled DNSSec is in your config and if your node is syncing, there is nothing more to do.