I updated to iOS 11.4.1 on my iPhone 6s (A1688) too and have been experiencing issues with the WiFi. Interestingly, I also updated on my iPad Pro (A1652) and that seems to be fine.
I'm using the Virgin Media SuperHub 2 in modem mode. I then have an additional router for better WiFi coverage and ethernet ports. Connecting to either router in either 2.4GHz or 5GHz seems to encounter the same issue.
I can't seem to find much information about it online apart from on this forum, but I think it's an issue with iOS rather than VM since my other devices (wired and wireless) are fine.
Well, VM Engineer called and installed a new hub (Super Hub 3). I've been on the hub now for over an hour and so far so good, the wifi issue hasn't yet reappeared but maybe it will return, I don't know. I have left the settings as standard so same SSID for both frequencies so that the phone stays connected for longer when I go out to the garden. I appreciate the speed does reduce when it reverts to the 2.4 GHz at that point but at least it is still connected. It regains 5 GHz signal when I return into the house, so all is good. Whilst he was here, the Engineer checked with his office if a free speed upgrade was available and it was so I have now gone form 150mbs to 200mbs, but I understand everyone is due the increase at some point anyway.
For completeness, the issue being experienced was with the following;
Super Hub 2 Iphone 6 Ipad Air 2 Iphone SE all iPhones/Ipad on iOS 11.4.1 Iphone SE upgraded temporarily to iOS 12 Beta 2 PR also experienced the wifi issue.
I wonder if not everyone experiencing the problem is related to what model iPhone/ipad is in use - maybe the older versions with different chip sets?
I will report back if the issue returns when the new hub settles down, but so far over the first hour all is well.
Since updating both my IPhone 6 and IPad air to the above O/S everything works fine apart from Internet/network drops approx every 10 mins fix disable and re-enable Wi-Fi easy but a pain I rebuilt IPad still same issue, Tested both devices on alternative non virgin router works fine also my home laptop no issues with home virgin router. Any ideas welcome please is it apple is it Virginmedia issue which ever those 2 devices not fit for purpose at home 4G fine Suggestions please