Time Machine-Silion Mac-Apple Airport Extreme - USB Disk Not Found

Two possible explanations.

Some mentioned that only the Airport Extreme 802.11AC supported external USB hard drives. Is that a correct statement?

Yes. For AirPort Extreme Base Stations Time Machine is only guaranteed to work with the latest production 802.11ac AirPort Extreme Base Stations — the "tall" variety. Refer to Backup disks you can use with Time Machine - Apple Support which makes the distinction abundantly clear.

There is extensive history on that matter, which began with Steve Jobs mentioning in an offhanded comment that TM could be used with any AEBS. It just wasn't so. Apple's Support documents never indicated it would. There was probably a lot of internal turmoil going on that we will never know about. No one wanted to say "you're wrong Steve" even though he was. In any event the introduction of the new "tall" AEBS models put the matter to rest. It was Apple's first officially documented and unequivocally supported non-Time Capsule NAS backup solution.

The 802.11n "flat" base stations may work with Time Machine. Lots of people reported that it did, but it was never officially supported.

It goes without saying all Time Capsule models work, all the way back to and including the original one. They work to this day. I still use them.

Next:

Is it the choice of how the security of the shared disk is selected?

Possibly. If you encrypted that backup disk separately, don't. It won't work. Erase it using the GUID Partition Map and Mac OS Extended (Journaled) because that's what the AEBS will be able to read. Then, when adding it to TM check the box to encrypt backups. That way, the backups themselves will be encrypted, as they should. Don't encrypt the entire disk. That's only useful if the drive is directly connected to the Mac.