Apple do not sell routers etc of any kind. All production of the Airport Extreme, Express and Time Capsule ceased in 2018.
Even if you try, Apple routers were never able to extend wireless of other brand routers.
WiFi Extenders are BAD. I notice Netgear is putting the word Mesh into the name now.. but that is pure fiction. It is not connecting by any mesh system. It is purely extender which means they will reduce wifi speed by 2 at a minimum.
And wanting to charge people $90 to try and fob you off is likely illegal depending on where you live.
What model is the modem? It is almost impossible to help you with products that are not Apple. My basic suggestion is to reset the current extender to factory and reconfigure it again very close to the modem router. No luck it is dead.
My suggestion in that case you be switch it to AP mode and run ethernet to the EX6150 and see if it can work that way.. if so you could use it with powerline adapters if you cannot run ethernet to your dark area.
OR buy a proper mesh set. e.g. TP-Link Deco M5 or M9 are popular here. turn wifi off in the modem and plug it to the 1st node of the mesh set. Units built from ground up as mesh work many times better than so called mesh extenders.
You can read the reviews here to choose the best cheapest mesh that fits your needs.
https://dongknows.com/best-wi-fi-6-mesh-systems/
https://dongknows.com/best-wi-fi-5-mesh-systems/
Modems are pretty much the same across brands so normally it will make little difference.
Final comment.. if you own the place.. wire it with ethernet. It will cost a bomb but work reliably for the next 20-40years, use no power, and will remain faster than wifi systems ever will be.
