PLDT contractors came to install the modem for my new 2099 Fiber plan in the second floor of the house. i'm Connected to the network and reached a speed of up to 300mbps on speedtest.com. I then used a 30-meter CAT6 cable to route it from that room to a TP-Link AX1800 router in the middle of the 2nd floor. Set it up as the main rohter and connected to the network. Reached the same-ish speeds as being directly connected to the modem.

I then routed a 40-meter CAT6 cable from the AX1800 to a D-Link DIR-600M router. Was only getting about 10-15mbps and enough searching led me to realize that it caps off at 150mbps and uses a 2.4ghz band. So instead, I routed it to a TPLink Deco M4 to see if that would make a difference. It improved a bit, but only to around 70mbps. I needed the internet to work in my grandmother's room for a meeting at around 7pm so I connected 4 other Deco units to the M4 (2 M4s and 2 M5s). I figure this might have had something to do with why it's slowing down but I can't seem to understand how to fix it. I disconnected the other Deco units and made a new network so the main one would be the only one in it, but the speeds still capped at around 60-70mbps.

Wiring all routers together is not an option for now and would defeat my main purpose of having the mesh system set up. There are no walls between these connections either.

Is the speed drop to be expected with my setup? Assuming the route from the AX1800 in the second floor to the M4 in the ground floor is the problem, what dual band router would you suggest?

In addition to this, LAN port 2 is connected with a 20 meter CAT6 cable to a TPLink WR840N on the first floor (I had no choice but to wire this one because this is an enclosed space with thick walls) which is also a 2.4ghz router. On the box it says 300mbps but I'm only getting around 10-15 on my phone and even worse on my laptop. Should I change this router too?

Sorry for the long post, but I hope this was comprehensive enough and straight to the point. TIA, guys!