Having exactly the same issue here too. I have even signed out of the account on the app and signed back in but it makes no difference. Looks like the app needs to be updated. It's unfortunate to say the least as I wanted to test out the hub 4 for wifi blackspots which Virgin media say you should do as the app can request wifi extenders directly with no need to contact VM customer service.
You can get a phone wifi scanning App to test the strength of your signals (and interfering ones) at any location. Alternatively, you can see whether your wifi devices connect and stay connected, to the wifi at those locations - if they dont - it's a blackspot!
As you have a Hub4 you are entitled to free boosters so they are worth giving a go (if you had tp pay £3/month - then they arent!). As you have discovered they cant be ordered by the App - so log into your VM Web account and you should be able to order them there. Or just call in and request them. Call at 08.00 is the best time to get straight through.
-------------------- John --------------------
My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.