So you have 2 of the Tenda AC1200?
If so, you can make the poor man's "mesh" the old way we did back before mesh systems.
Keep the main router as it is. No changes.
Turn on the new router and connect a laptop or other computer to it via a network cable to the LAN port.
Use the computer to log into the management page of the new router.
Change the router LAN IP to a slightly different IP address than the main router. If the main router LAN IP is 192.168.1.1, make the new router's LAN IP address 192.168.1.254 or similar.
You'll need to login to the management interface of the new router after that IP address change (use the new IP).
Once you login, find the DHCP server settings in the LAN settings area and turn OFF the DHCP server. Save the settings.
Now connect a cable from a LAN port on the main router to a LAN port on the new router. Make sure the cable is long enough to reach where you want the new router to be located.
Configure the wifi settings on the new router to match the ones on the old router. Same SSID (network name) and same WIFI password. Save the settings.
Now you have a second wifi access point that is connected to the existing LAN from your main router.
Because the system is no a true mesh where the two wifi access points know about each other and manage client devices together, you may have trouble where moving from the range of one access point to the other will be fiddly or slow (it will take a while for your device to 'let go' of the old access point and use the new one, or vice versa, as you move around). Should be ok though.
EDIT:
Actually, never mind. If you have 2 of that same model, look at the manual. There is a section on how to make one of them mesh with the first. Give that a shot.