I've not had any problems using iOS 12 since it was installed a couple of weeks ago. I've used the phone on a variety of wi-fi networks (here and abroad) and this current problem is the first time it's played up.
I've had lots of problems with the BT Smart Hub 6's stability - it goes down at least twice a day, sometimes a lot more. I've put this down to the DLM lowering the SNR to a level that's too low fow my ADSL line (I'm over 3km from the exchange). This has been going on for nearly a year and I've given up trying to get BT to sort the issue out (4 engineer visits, lots of resets, swapping filters, using the test socket etc. etc.).
Anyway, I swapped the offending BT Smart Hub 6 for one that BT replaced back in January and the performance has bounced back up again. Let's hope it stays that way.
Now, how do I convince BT that the wi-fi problems I was having with 2 iPhones but nothing else is a problem with the hub?
Glad to hear that things seem to have been resolved for you. Connectivity issues have been raised on the Apple developers community, but reports are sketchy some refering it to certain handsets and some to firmware. I am actually having my handset swapped out on Friday to see if this resolves my issues.
If you have exhausted all routes with BT, then make a complaint Make a complaint
The smart hub does not cope well on long ADSL lines as its more susceptible to interference which is out of the normal ADSL frequency range, owing to the fact it has a much wider bandwidth.
Its not possible to alter its configuration to select ADSL only, like many other routers.
The home hub 4, or a third party router, is the best option for ADSL.
If its configured for ADSL, and you can see the DSL synced, then all you should have to do would be to enter the default login details on the PPPoA setup screen, if there is one.
You can use a user name of firstname.lastname@example.org and a password of bt
Dynamic IP and DNS servers instead of static IP. You may be best to factory reset it first.
Before doing as Keith suggested you should carry out a factory reset on the router which will clear out any settings that you may have had from your previous connection.
EDIT: Just noticed that you have now suggest that Keith!