Numerous posts in this community say that it is possible to connect the RBR50 and RBS50 (AC3000) routers and satellites by wired backhaul to the RBR750 (AX4200) and use them as satellites to extend coverage.  After purchasing the RBK752 package and failing to succesfully connect them, Netgear now tells me there is no connection possible, wired or not.

Very dissappointed: my AC3000 units were functioning fine.  I wanted to extend my network to a separate building (with 200 feet of buried Cat 7 cable) and could not find another RBS50 satellite so I upgraded.  Now may be forced to buy another RBS750 satellite, spending hundreds of dollars I apparently did not need to.

I guess buyer beware.