@Sunshine Thank you for your reply! Now I can say that, after the firmware upgrade as suggested, the router is working fine error free since 3 weeks! The only question I would like to Address now is...

@Sunshine Dear All, I have the same issue and also, by checking the label on the router, I see it's an Archer MR550(EU) Ver: 1.0 while, checking the device "Advanced > System Tools > Firmware...