Make RBR50 as satellite

Here is my issue:

In last 2-3 weeks my internet has been very slow (my usual download is 40Mbps). I have an ORBI RBR50 and Satellite RBS50 since 3 or 4 years (not sure how long).

My main router is downstairs in family room (no basement), Satellite is upstairs in main bedroom. My older's son bedroom is opposite of our bedroom and the main router. His access drop to about 5-10 Mbps.

We had many issues in last 3 weeks about accessing anything however our internet access remains at 35-45 Mbps when tested at the modem.

I'm thinking to get a 3 pieces ORBI RBK23 (and 2 satellites); I know I can probably pair my current satellite to this new system (I assume), but could I make the the old Router a Satellite in the new system?

It will allows me to put 1 satellite in each bedroom and use ethernet plugs in the back to plug my kids Xbox and Computers.

Is it possible? All Satellites will probably be about 30-40 Feet from each others.

Thanks for helping, I'm getting frustrated (and my family even more) about this situation.

Thierry

PS: My current system is running firmware 2.5.1.16 and I have about 20-25 equipments on WIFI.
Model: RBR50|Orbi AC3000 Tri-band WiFi Router