Building an extended home network with Netgear

Hi!
I tried but was not successful in finding information on how to extend my wifi home network with either MESH, extenders or secondary router(s). The key point is that my house has three floors and concrete structures are challenging for 2,4 and 5 GHz RF signals. So, my main wifi router only covers one part of the house, while there is no network connection in other parts.

I bought a Netgear LBR20 LTE router, which I use as my sole WAN connection. There is a 4G base station not too far, and I get a nice 150 Mbps download speed to the LBR20. I have ethernet cabling from the LBR20 location to a switch and from there to other parts of the house, so I can get a wired connection. 

If I want to extend my wifi coverage to other parts of the house, what would be the best way forward? If I understand this right, I could:

a) buy a cheap wifi router and connect it to the switch over fixed ethernet connection and set it up as an access point without routing (=bridge mode). Would this work with my LBR20?
b) buy MESH routers and extend my wifi coverage with them. But I have seen that the "MESH-master" should be configured to act as the DHCP server and main router in the network and the "MESH-slaves" are then just APs connecting to the master over either wifi or fixed ethernet.How should I configure my LBR20? 

When looking at MESH devices we have a bargain deal for two Orbi Pro AC3000. But will they work with my LBR20? 

And how should I connect my LBR20 to the next MESH device? Can I use fixed ethernet, or does the MESH devices need to use the backhaul link to connect to the main router?

All advice is greatly appreciated. 

PS: Of course I'd like the wifi in all locations have the same SSID and that I could roam seamlessly from one floor/location to another. 
PPS: What is the logic with Netgear model naming? My device is Netgear LBR20 Orbi 4G LTE Cat.18 WiFi but there is no such model in the top bar "Model (Recommended - Helps the community give the best answers)"