Cannot synchronize with Bitcoin in testnet mode
I got it to work by adding
[test]  
addnode=104.237.131.138  
addnode=151.80.205.132  
addnode=192.155.82.123  
addnode=74.220.255.190  

to the config file.
The reason is your wallet needs to connect to nodes to connect to the blockchain. Usually it comes with all the nodes required to connect, but sometimes the devs failed to include the nodes or released a new node list after an update (try looking in the bitcointalk page or the official website listed by the coin developers), so you have to add them manually.