RBS50 satellite connect to RBR850 network?

I have been running RBR50 + 5 satellites for a while and it have been pretty solid.

i just picked up an RBR850 and 2 RBS850 satellites, but range is not quite enough for the whole house, I need an extra satellite.

I did a quick ‘chatgpt’ which suggested I could use the RBS50 to extend the range, saying that particular node would be slower, but it would at least give me the coverage.

Anyway, I tried a number of times to connect it,  without much success. On one occasion, I thought it had worked, but the SSID was still the factory default, not the same as the main mesh network. Although I could connect to it, it’s not really a solution 

Am I wasting my time ? Do I need to fork out for another RBS850, or can anyone suggest other things I can try?

TIA