APFS is certainly no good.. so other than disks directly connected to the Mac running High Sierra.. forget it.
The airport can be kinda specific about what disks it reads and what it doesn't.
Also High Sierra... can be a dog to network drives.
First things first then.. does the disk show up under the disks tab in airport utility.. no disk there.. then it is unrecognised.
eg.. plug a disk into my current older TC..
If the disk fails to show up.. remember no matter which airport you have it only runs USB 2.0 port. Some USB 3.0 disks have chipsets that simply fail to be recognised.
As Niel mentioned the powered USB hub is a good deal.. USB 2.0 btw.. it offloads the power demand.. a good thing.. as well as sometimes handing recognition better.
If it still fails.. Sorry but there is just not a lot you can do.. Plug in a USB memory stick to make sure everything is working as it should..
Also note speeds are very poor.. USB 2 on a router will be lucky to achieve 10MB/s read or write.
Are you running Gen 6 AE.. ie the AC wireless tower model?
Earlier ones do have bigger issues with power demands of self powered USB.. is your USB drive powered or not?