@MR.S 

unfortunately no, the ISP is not providing the internet with RJ45 or Fibre.

  @lkakas 

I don't have any more suggestions for you, so we'll probably have to wait for TP-Link.

  @lkakas 

If it was me facing this, i would now start enabling other ports as WAN and trying to connect on say WAN/LAN 5 instead and see what happens.

I know you say you have checked the cable, can you try with yet another one as well?

  @GRL 

I did that as well moved to WLAN/LAN6 with a 3rd brand new cable same issue :-( Webpages are loading incomplete, downloads interrupted or not even starting.

Connected an old TP-LINK 1043ND to the same modem works without a problem.

Next I'll try to upgrade to beta firmware.

EDIT: same with beta firmware 1.2.3.

I'm out of ideas.

  @lkakas 

Do you have IPS / IDS turned on, or any firewall ACLs that control traffic in or out of the WAN connection that may be interfering ?

Other than one very specific WAN port bug (reported and ticketsed that is nothing like yours), my ER8411 has been flawless on internet traffic with 4x WAN connections (All RJ45) running simultanoeusely serving 10 vlans across an extensive business network.

  @GRL 

No IPS/IDS enabled, firewall yes, but I did a factory reset and with default settings doing the same beaviour.

  @lkakas 

Do you have a spare, dumb unmanaged switch?  Would be curious to see what happens if you put a switch in the middle

Modem > switch > ER8411 WAN

  @GRL 

I put a switch in between the modem and wan port -> results are the same :-(

I tried to play with flow control but also did not help.

  @lkakas 

Realy not sure then.  Could be a faulty unit i guess but you will need to raise this with TPLink direct.

One of their team will reply to you im sure in a day or so.

  @MR.S Thanks, really appreciate your efforts guys.

Despite I see no error when connecting to the ISP modem directly with my laptop, I'll ask them if they can see any issue on their side while I'm connected with my laptop through the ER8411 to the modem.