If you don't mind using an external device like a Raspberry Pi zero I made an external bandwidth monitor. Instead of probing one router you probe all three routers and get the total. You need to modify router.php file and add three requests and add the final total to the variable. The best part is that you can reboot your routers any time and still get pretty accurate data.
Link to the project for more information - Made an external bandwidth monitor for OpenWrt