Your modem does not have WiFi right? For comparison test, please replace the AX1800 with the other router instead of piggy backing to it.
For the 2.4GHz band, 0.08Mbps means it’s not working at all. Can you do a ping test to see the % of packet loss?
What speed are you getting with the 5GHz band (in Mbps)?
Also, please do a speed test with Ethernet connection to a computer. That’ll establish your baseline speed.