What is the model of router and version of OpenWrt?
Perhaps post contents of /etc/config/wireless (remove security keys). Use </> to format the text.
DCP-T720DW only seems to support 2.4 GHz considering the model is not particularly old. Is the firmware up to date in the printer?
If you can't get the DCP to connect, you may have to install a separate wireless access point running stock/OEM firmware for better wifi compatibility. (I personally find 2.4 ghz openwrt wifi to be temperamental)