Problems with connections to Teamviewers

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Hi guys, I'm facing some stange problems using Teamviewers recently.

I have set up a new site with 100M/100M FTTB and using Draytek Vigor 2926 Router.

There are 4 PCs connecting to 2926, and those PCs were using Teamviewers working properly on another site before.

But when I try to connect to those 4 PCs now, it always stays at Initialing window for 3~5minutes(which only takes 3~5 seconds before), sometimes it even need to reconnect.

I already discuss with DreyTek support engineer and disabled all the firewalls and DDOS settings, also adjust MTU of WAN from 1500 to 1412.

When MTU set to 1500, all 4 PCs show up in online list in Teamviewers, but can't connect to them properly, always hang at initializing and reconnect window.

When MTU set to 1412, all 4 PCs show up in online list in Teamviewers, and it takes 3~5 minutes to connect to 1 PC as describe at the begining of this topic.

I have no idea what's happening now, the WAN bandwidth is more than enough and not much loading on Vigor 2926.

Has anyone faced this problem before? Any suggestions on how to solve it?

Thank you for all the kindly help and suggestions.

Have a nice day : )