I think that W8816 will be good solution for bridge mode in your network.

Poor line causes that modem inside your TP-linka reduces the speed of the internet to gain a more stable link (lack CRC). Maybe your old device has better ADSL modem. In my experience I know that...

Can you show us screen shot from status tab of that router?

I think that default port will be 80.

Sorry I don't know that ISP. I'm from Poland :)

What kind of service or server do you have on this 30000 port? Is this service/server works all time?