The EM7511 has PCIe Interface and you used a PCIe to USB adapter, right?

As far as I know Sierra uses a different driver which is not compatible with current firmware. Your other openwrt may have compiled the driver in.

The driver could be supported and I need to ask expert to have a check. If you could send logs that could be helpful. The output of the following command:

dmesg
logread
ifconfig -a