@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...

@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.

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

@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...

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

@MR.S OK , I did factory reset. Then test without any specific settings. Same problem. restored config , same problem.