Sir, since Apple has discontinued these products (Network related product, I suppose?), is it possible to buy another network drive (without the HDD), and plug my current HDD, and just resume using the backup in “sparsebundle.”
I'm not going to say that it is impossible, but having tried this in the past and failed after spending hours with the support folks who all said it "should work", I would not recommend that you try this.
Or can I plug my External HDD to the USB port of my router, and back up the same way I did using Airport Time Capsule? - sorry if this sound too amateur
Yes, if your router is an Apple router. And yes, if it is another router by Synology or Asus, since these products support Time Machine to a USB drive connected to the USB port. Most other routers will not support Time Machine backups to a drive connected to their USB port.
I hope Apple can develop a software to help Airport TC Time Machine users “backup/migrate” to local backup with ease, because I am sure all Airport TC users will encounter same problem sooner or later as the Airport Time Capsule ages.
Unfortunately, Apple has done virtually nothing in the way of support since they discontinued the Time Capsule and other AriPorts in April 2018, so......in my opinion.....I would not expect them to change their ways.