Browser-router interaction?

My mobile phone has Wi-Fi turned off. I use the mobile signal if I want to connect to the internet as I have a good signal here.
Yesterday I was at a house where the mobile signal is poor so I asked if I could use their router. I was given the password, and turned on Wi-Fi. The phone found the Sky 829CE router and connected to it. I started browsing using Firefox, and had no problem. I then used Brave and that was fine too. However, Vivaldi would not connect to the internet. I can't remember the exact error message, but I think it said I should check the internet connection (not very helpful when you obviously can't connect!), or check any VPN settings. I don't use a VPN. I've been using Vivaldi for a couple of years without problem, so couldn't understand why it couldn't connect to the internet. Both Brave and Vivaldi are Chrome-based, so why should one work but not the other?
Several other apps had no problem connecting (FairEmail, BBC iPlayer, Netweather, etc), so it appeared the issue was with Vivaldi. I uninstalled it and reinstalled via Play Store (no problem with that connection either), but it still gave the error message. As Vivaldi had always worked before using the mobile signal, I turned off Wi-Fi and forced it to connect via the mobile signal again. It was slow, as the signal isn't great, but it worked! I was able to browse without problem. When I turned the Wi-Fi back on I got the internet connection problem again.
I'm back home now but when I next visit the house I'll try Vivaldi with Wi-Fi again to see if it works ok or not.
Any explanation for the strange behaviour?
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Jeff