I am somewhat confused by what you mean "plugged into".
If you use a ethernet cable from your pc directly into your main router you should get more or less what you pay your ISP for. It is possible to get a little more than 900mbps on a 1gbit plan. If you take a ethernet cable and connect it between the main router and a "mesh" unit and then connect a cable between your pc and the mesh
unit you still should get the maximum rate. The mesh unit is acting as a simple switch in that case.
If you are connecting between your main router and the mesh unit via wifi and then you plug a pc into the remote mesh unit with ethernet all you did was buy a expensive nic card. It really isn't much different than a nic card you connect with a USB cable.
You still have a wifi connection in the path and it may or may not be faster than a internal wifi nic card in your pc. Things like PCIE nic cards many times use the same wifi chips as the routers. Problem is the antenna are located very near the metal case in a lot of cases.
There is no magic device that can exceed the government limits on output power. In this case it was easy to get the fccid of your units. Seem they are made by the same company that makes tenda and then they sell them for almost double the cost of the tenda units with the same exact fccid.
Although you likely have never looked at these reports before the summary part shows the output power is very close to the maximum level of 1watt of power. If you were to look up other brands of routers you will see numbers very close and small differences don't matter. Something as simple as a piece of paper in the path can drop 100mw off the signal levels.
I am somewhat surprised you get more than 300mbps on these units. These are using what most end device are which is a 1200 "number". These are big lies. First they add the 2.4g and 5g speeds together even though a single device can't actually use both. The 5g is actually 867 even though they round this up to 900.
Next they are adding transmit and receive speeds together. This is like calling a gigabit ethernet cable 2gbit. Ethernet cables unlike wifi are full duplex and can actually get those rates. So you cut the speed in 1/2. Next there is a lot of overhead in the transmission so your end user data can't use all the bits.
So even directly next to the router you might get say 400mbps. This just gets less and less the farther you get from the router and the more wall you have. It is fairly common for people to get about 300mbps on wifi with maybe 1 wall between.
Now if you were to use the highest end wifi6 devices that support 160mhz radio channels you might get numbers closer to 600. This is purely because they are using more radio bandwidth not because the signal goes farther. In fact it goes less unless the router drops back to a simpler data encoding as you get farther away.
If you can get 300mbps on the second floor be very very happy. That is much more than many people can get.