Yes it's being run on the testnet, I have no other nodes running.
I have no firewalls as far as I know, I'm running Ubuntu with default settings and I even tried disabling ufw.
I configured the config file to according to the book "Mastering bitcoin" as so:
rpcuser=bitcoinrpc
rpcpassword=(arbitrary digits I chose)
I'm using an external harddrive as datadir.
Btw, when using an external hdd as datadir I have to run bitcoind as sudo for it to work.
I'm downloading the testnet to my internal datadir as I'm writing this message, don't know if it will fit though.
I haven't tried the mainnet because the blockchain is to big for my internal hdd.
I don't particularly know how testnet works/where you're supposed to get the data from?
However, try running it with the mainnet with the command -prune=1000. The number is the size the download data is limited to in megabytes. Minimum is 1000MB if you have enough internal space free.
Otherwise, where are you downloading the testnet from? Is the reason it can't connect because there aren't any nodes with a compatible blockchain.