My current setup is as follows: I have a bsnl broadband connecting to an Iball ibaton 150M wireless-n adsl2+modem router (@192.168.1.1) in bridge mode, and an archer c6(@192.168.0.1) in router mode setup in PPPoE. the archer c6 covers the first floor of my home. I have run a cat 6 cable from the archer c6 to another Iball modem/router(@192.168.0.2) (same model as the main modem in bridge mode) on the ground floor which is in AP mode (DHCP disabled).
I have configured both the archer c6 and the iball in AP mode with the same SSID, password, and security protocols(WPA2-PSK, AES) to have a seamless handoff when I move around while using my phone. I'm using an Oneplus7 pro.

here's the scenario. when I'm connected to the archer c6 when I'm in the same room as the router, it connects to the 5 GHz band. when I leave the room, it will switch to 2.4 GHz. and when I go downstairs, the phone switches to the AP downstairs seamlessly. Now, if I come back upstairs, the phone switches to the 2.4 GHz and then to the 5 GHz as I enter the room without any hurdles. The problem is that after the phone switches to archer c6, i lose proper connectivity. I have some connectivity as in I can send/receive texts or browse the web, but it's like something is messing up the bandwidth. youtube doesn't load, cannot make video calls and a speed test at this point gives me 0.07-0.1 Mbps in place of 8.2 - 8.3 Mbps download speed I should get. I have to wait around a couple of minutes before I get proper connectivity. I don't know exactly how much time i have to wait before the connection is okay again. But if I go downstairs again during this time and the phone switches to downstairs AP, the network is fine. i had written to tplink support couple weeks ago but i still haven't heard from them.