@FidoPierre, @KAPABLE-K, thank you for your replies. I tested both with hardwired LAN to my pc and also ac wirless at 5Ghz. Both the modem and my router were hard rebooted. all signal levels shown in the modem page are quite close to optimal. I tested again early this morning the problem still existed but a few hours later it's gone on its own. So not sure what happened. Some tests pictures below.
Problem can be considered solved for now, hopefully it won't return any time soon!