Sorry don't know what to suggest after that
Maybe disable qos(check there if is any setting sill on) Bandwidth Management >> Quality of Service
DoS and un-tick the option for priority to voip(this is new feature)
also do add manually at internet access the adsl2+ or something in that section if is only the BE connection or is just bridged pppoe?

mine was fine with 3.6.0 but xmas when i will have time off for 3 days i may put back mine as well (chance to upgrade it- i see many improvements)

Updated mine with 3.6.3 and all fine
Just a small report

Seems it struggle little with the speed.I do get 100mb but i can see the speedometer struggles and sometimes will not reach fully

also if will turn ipv6 off there is not access to wan> internet access page and giving error 500
When turn on ipv6 is fine and wan>internet access working normal

As for your issue did you ask the support what they suggesting? can try & the utility firmware upgrade tool