Help needed with ORBI issue

Bought a RBR50 Orbi AC3000 plus a RBS50 Orbi Satelite. Set up both according to instructions and both worked as expected, with a few performance issues. Did not maintain a record of the original FW installed. As the web interface suggested a system upgrade, clicked on it and got an upgrade to v2.5.1.8 both on the router and the satelite. The router wi-fi went into intermittent mode, coming and going every 10 or 20 seconds. The satelite just did not connect any more.  

I downgraded the firmware all the way to 1.4.0.16. Intermittent wi-fi continued on the router. Went up through all the versions of firmware, same results. I only managhed to pair the Satelite when I got back to v.2.5.1.8. Each time, I went through the lengthy process of resetting to factory default and reconfiguring everything. But what with an intermittent wi-fi, it was useless.

BTW, I do have an ISP router, whose wi-fi has been turned off. My optical fiber network ISP is 240 Mbps nominal (what with Covid-19, sometimes I get 2 to 5 Mbps nowadays). I do have double-NAT, since I need specific addresses on my home network (because I hav IOT devices which would be painful to re-address).

Have given up and gone back to my previous Router.

Shall wait until Netgear manages to produce a stable FW version for the RBR50/RBS50.