Xiaomi Mi tv stick 4k losing Wi Fi from secondary (access point) router
Hello folks.
I have a really odd situation, made only worse by the fact I am no tech guy so I have no clue what to do.
I recently bought a Xiaomi Mi tv stick 4k in order to turn an old TV into a smart TV.
It's parameters are Wi-Fi 2,4GHz/5GHz, 2gb ram, Android TV 11 as OS.
Now I have two routers. One is the main router hooked up to my modem and PC. That is an Archer C6U dual band1200 Mbps. To it I have hooked by cable a second old router as an access point. It's so old I can't be sure of the actual model, but by looking it up I assume it's a TL-WR740N. One of those old, white ones with just 1 antenna.
I've actually had them hooked up like this for over a year. The access point I made as online videos and instructions from forums here said I should do (change LAN IP, disable DHCP server, etc.) and it worked OK. Phones and laptops have been connecting to the AP router without a problem for months. Then I tried to hook the new TV stick to it (the AP router) the other day and the weirdness began.
The Mi TV stick connects, has Wi Fi for anywhere between 2 mins to 30 sec and loses it. If I go and restart the router via the power button it finds it again and the occurrence repeats. Oddly enough when the TV stick loses the internet, seemingly, all of the other devices hooked to the AP router, like my phone, lose the WiFi as well.
The main router and the PC at the other side of the house are fine tho.
I thought maybe the stick is busted but if I hotspot it with my phone's own cell data, no problems. I tried hooking it to the main router, but that thing is so far away the signal is incredibly weak. You can't be sure if the tv stick is loosing the Wi Fi due to the problem or the 3 brick walls between it and the Archer C6U.
I tried some weird things yesterday like trying to reserve the stick's IP, making it have a static IP (idk if I did it properly). Reset both routers to factory settings and configure the master and secondary router as an access points with various LAN IPs for the AP router (I probably factory reset configured the thing 7 times with every suggestion I could find in every forum or YT video comment section I could find). Nothing. I factory reset the TV stick, again nothing. Checked for updates on the Android OS, downloaded them, again, nothing.
1-2 minutes of wifi and then gone. But if I don't actually use the TV stick, if it's just plugged in the TV, the AP router seems to work normally.
So... any ideas?
Thanks in advance for the help.