Sporadic Success Opening SSL VPN Connection through C7000-100NAS Gateway/Router

I have had the same ISP (Comcast Xfinity) and gateway/router (C7000-100NAS) set-up at home for about two years and have been very happy with general performance. 

 

From my laptop I access an SSL VPN using DrayTek SmartVPN Client for work. Until a few weeks ago this was performing very well.

However, now, when I try to connect to the VPN from home, roughly 90% of the time the Smart VPN Client gets stuck on the "Open SSL Connection" step and I get "Can't connect to SSL VPN Tunnel server" message. 

 

The VPN Client retries over and over and usually eventually manages to establish a connection but it can take many attempts (typically between 2 and 20).  I would estimate that on average it takes about ten attempts before successfully connecting.  Once connected, the connection is typically stable and I can access and use the remote server without problems.  

 

I can access the VPN from my laptop at other locations including Starbucks, public library, friend's house (who also has Comcast Xfinity ISP) and in all cases the connection works first time, every time.  So I am guessing that it has something to do with my gateway/router.  I had not changed anything on the gateway/router when the problem initially occurred.  I have experimented with some of the settings but with no success.

 

I am out of ideas so reluctantly considering purchasing a new gateway/router but would appreciate any guidance the community can provide to help troubleshoot the problem.  Thanks!