Added RBS350 to RBK752 resulting in no 5GHz network on the RBS350.

My home wifi setup is RBR750 and RBS750, which give good wifi coverage throughout the house except for one room where the speed dropped to 2-12Mbps. After learning that RBR750 is compatible with the budget friendly RBS350, I decided to add it to this room by wiring it to the RBR750. In general, everything works except that the RBS350 does not attach devices in the 5GHz band, only in the 2.4GHz band. On the status page of this satellite under 5GHz, my home SSID is not listed, but some hidden SSID is. I understand this is a wifi backhaul with the router.

RBR750 and RBS750 firmware is V7.2.6.31_5.0.24
RBS350 firmware -– V4.6.14.3

Please advise:
1. Is there a way to get working 5GHz on the RBS350?
2. Is it necessary to change the whole setup to firmware version V4.6.14.3? And can this firmware be used on RBS750?
3. Is it possible to disable wifi backhaul on RBS350 since it is connected via ethernet anyway?

• router RBR750

• satellite RBS350