The Balance 20/30 (hardware revision 6 and below) has been discovered the USB WAN connection will cause system instability issue after upgrading to firmware 8.1.1GA (8.1.1 build 4986).
We have prepared the fix with special firmware (8.1.1s017 build 4987) to address the issue. You can load this firmware into your affected unit(s) that need to use the USB WAN.
Balance 20 HW7 is NOT affected, hence, is optional if you want to upgrade with this special firmware.
[Update: 27 January 2021]
We received several cases reported that some USB dongles may not work well with 8.1.1s017, hence, we have prepared 8.1.1s048 build 4990, with added enhancement to the 8.1.1s017.