Chosen Solution
As mentioned, the wifi from the wireless router (fibre modem + separate router- 2.4ghz only 20mhz )does not connect to the macbook. Same happens with my mobile personal hotspot. It only connects when the router is at max only 5 cm away. i get good speeds when it is connected but even if i move 10 cm away, the Tx rate drops from 73 to 1mbps and the connection becomes lost after some time.
I had this issue on a Macbook Air (2012), and replacing the wifi card solved my problem.
I often find the cables get damaged! The routing of it is a bit tricky. Review Step 30 in this guide MacBook Air 13" Early 2014 Logic Board Replacement A damaged connector, deep kink or worn area on the cables will require a new antenna unit (to get a new cable) 13” MacBook Air Hinge/Clutch LH - iSight/Antenna Cables, Apple P/N 661-02397