Terry Hardie

I've spent around 10 hours on this issue. I used the most recent rockpi image for Armbian (Armbian_21.08.1_Rockpi-4b_hirsute_edge_5.13.12_xfce_desktop.img.xz) 
 
I managed to finally make it work by adding this dts file:
/dts-v1/; /plugin/; / {         compatible = "rockchip,rk3399";         fragment@0 {                 target = <&usbdrd_dwc3_0>;                 __overlay__ {                         dr_mode = "peripheral";                         compatible = "snps,dwc3";                         reg = <0x0 0xfe800000 0x0 0x100000>;                         phys = <&tcphy0_usb3>;                         phy-names = "usb3-phy";                         phy_type = "utmi_wide";                         snps,dis_enblslpm_quirk;                         snps,dis-u2-freeclk-exists-quirk;                         snps,dis_u2_susphy_quirk;                         snps,dis-del-phy-power-chg-quirk;                         snps,xhci-slow-suspend-quirk;                 };         }; };
So, write the above to dwc3-0-device.dts and then run:
sudo armbian-add-overlay dwc3-0-device.dts
After this, loading g_ether or g_mass_storage worked fine.