I've been having the same problem since I started trying to using Bitshares DEX a few weeks ago. Currently 7 (!) nodes are down, 1 is showing neither up nor down (Public Testnet Server) and only 3 are up, but they all have low latency. One again the DEX is completely unusable!
they are not down, it just took your browser to long to ping them, basically.
check if you're using a bad VPN or firewall. if not, then just connect to one that's shown as down. it'll work.
or try with an alternative wallet
I use DEX every day, and give a lot of support. most of the times it's a local issue