RAX200 (AX12) weaker & slower than R7000 - (I've lost 70Mbs and WiFi is plop)

What wireless card is in the mac? 

Have you tried changing the channels the rax200 is on? Maybe theres interference on one. 

have you tried speedtesting on each of the 2x 5ghz bands? and not just one? 

What speed do you have set for the rax? is it set for "up to 4800mbps"

If your area is not congested, on the WiFi settings, uncheck the 20/40MHz coexistance setting for the 2.4GHz band. Newer netgear routers have been more willing to drop to 20 MHz channel width when overlapping APs are detected. While it is useful if the overlapping APs are highly active. If they are not very active where you will end up with a ton of airtime sharing issues, then you will notice a near doubling of the performance on the 2.4GHz band.

For 5GHz, it should be significantly faster, The RAX200 with a 2 stream 802.11ax client, is able to deliver a real world throughput of 1.8Gbps to 1.9Gbps on a 2.4Gbps PHY rate. Furthermore compared to the R7000, it also has a significantly higher transmit power on both the 2.4GHz and 5GHz band. Beyond that, one area that Netger does well in, is improving receiver sensitivity and minimizing self-noise, thus maintaining higher speeds across a given range (less of a dropoff), thus even in those cases, performance still improves.

The one exception, is if you use DFS channels, If you do not have any devices that support 160MHz channel width on the 5GHz band, then change the WiFi mode from "Up to 4800 Mbps" to "Up to 2400 Mbps", this is because in the US the FCC unneessarily restricts any transmissions that enter the DFS range to a max of 250mW instead of the normal 1000mW. That will cause the router to use 80MHz channel width on the 5GHz band, which will allow it to avoid the use of DFS channels.

Depending on the AP, sometimes signal strengths can change, e.g., if you move from a model that has 2dBi antnnas, to 5dBi antennas, then clients a floor above or below the WiFi router will see a small drop in signal strength. Furthermore signal strength can be unreliable of some lower end WiFi adapters if very close to the router, since some lower end WiFi radios handle receiver overload rather poorly.

OP, I wonder if you've received a lemon RAX200? I'm also using a RAX200 on a TalkTalk FTTP Fibre 900 line and the RAX200 is the best router I've ever used. I'm getting 800+Mbps over wifi 6 almost anywhere in the home (new-ish double storey) and even wifi 5 clients give me around 600 Mbps. These are the settings I'm using:

RAX200_2G: channel 6, up to 1200 Mbps

RAX200_5G1: channel 48, up to 4800 Mbs

RAX200_5G2: channel 157, up to 2400 Mbps

Mu-Mimo ON

Enable OFDMA ON (all)

Enable Beamforming ON

Try the above settings.