There is a school of thought that says the VM Connect app is (like most corporate apps) a bug riddled P.O.S. I couldn't comment because I wouldn't bother to install it on my phone, and indeed it wouldn't work if I did because I use my hub in modem mode.
Which, I'm afraid is my recommendation to you, and involves spending a few quid to banish wifi woes forever. The wifi performance of VM's hubs is weak, and despite years of complaints the company continues to offer even its newest hub with a poor spec, so there's no mileage in asking for a Hub 4 (which isn't even on general release yet). Even if you get the Connect app to work, it'll just tell you you need to order one or more of VM's over-priced and clunky boosters that you have to rent for an extortionate price.
Far better to buy yourself a mid price mesh wifi system, put the hub in modem mode, and that should resolve the issue. I use this, and it's excellent, probably the best seventy quid I've spent on tech in many years. There's more expensive systems from Asus and Netgear if you want to roll the boat out. Avoid anything that doesn't have gigabit ethernet ports, and avoid Amazon/Eero, Google/Nest and BT systems - they all work, often very well, but are comparatively expensive for what's on offer.
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