Off the top of my head, have you set up static routes correctly? I haven't dealt with Omada before, but you should have a rule sending all traffic to your VLAN subnet to its specific interface, eg switch0.10 or whatever

You don't need ax for those kinds of speeds. An ac dongle, or even n if you don't have much interference around (assumedly so as you're in a house), would do just fine. Of course it wouldn't hurt