Explanation of wifi set up

I have an iMac and MacBook Air which both connect to internet wirelessly.
If I use the WD My Passport Wireless Pro (MPWP) to provide continuous back up wirelessly via Apple Time Capsule then the internet connection for both the iMac and the MacBook must remain set to the internet access through the MPWP? Is that correct? My download speeds drop by > 50% if I do this.
I realise I can use the MPWP as a dumb external drive but that switches off the wireless capability.
Presumably if I have a wired ethernet connection for my iMac then I can use the MPWP wirelessly without reducing my internet speeds?

NO! just connect the MPWP to your local LAN and connect the iMac and MacBook back to your routers Access Point.

Done that.

Using apple time capsule backup to the MPWP( by usb), now connected by home Wi-fi I can see the MPWP from my desktop browser but the time capsule app can’t see the my passport to do another backup.

Bizarrely I can see all the backed up files opening time machine.

Any thoughts?

Jonathan Shribman