Are you trying to connect from the same network or from different networks? If the former, try to connect to a different network (if you are on Wi Fi, switch to 3G/LTE, or vice versa) and try the same.
Also, could you send a screenshot of your mobile app's server settings window? You can send a screenshot via a private message or to firstname.lastname@example.org
After starting the portable viewer (6508), i see my list of available computers. I'll pick one of them and it's shown as "online" - everything normal until here. Just after clicking on a computer (to open the remote control), it's shown as "offline" for about 5 secs. The connection will be established and i am able to see the host screen and do normal operations. Is that normal? Sometimes it delays for more than 5 secs...
Regarding this issue. Found a small bug that caused this behavior. So it was not normal, actually, I was mistaken. So we'll fix it in the upcoming version.