Better router.

I went with HomePlug method many years ago. Reliable if power is single phase. If three phase, very spotty.

Decided to upgrade the router. The Asus RT-N66U was great at first and then after latest firmware, performance worse than a cheap TP-Link router. Retire it and used Apple Airport. Expensive but reliable for few more years.

This year, replaced all with D-Link (can't remember which model but has 4 antennas) and immediately, my network is stable again. For downstairs, I added a Netgear EX-6200 and set it to transit at full power. Now my 2.5 storey house as full internet coverage with just 2 devices only. No more HomePlugs or APs everywhere.