Here I helped someone with a problem like yours:
GL-AX1800 Flint with Sonoff DW2 Wifi - Technical Support for Routers - GL.iNet (gl-inet.com)
You can see that there’s a pace where to enable legacy 802.11b rates. I don’t have a Flint myself but you could try to search in wireless page on Luci, or downgrade your firmware to 4.2.3 if you can’t find this config.
Also, you could try the IGPM snooping config like the OP did.