I have come back here and registered specifically to try and give some hope and a solution to people who (like myself) have been having really weird 'sticky' internet with Three.
E.g. you'll be browsing fine, then suddenly it just starts to refuse to load pages. Often until you restart but that doesn't always fix it.
Here is what I have done.
Firstly, I got a TP Link Archer Mr6 router. This took my speed from around 12mbs to 40mbs and 4G+. Great I thought, all sorted.
Nope, same issue, but now I can get faster speeds between the issue happening.
Here is where I hit gold though.
I have a Linksys WRT3200ACM sitting about. This happens to be one of the routers that ExpressVPN release their own software for.
I have now put that router between the TP Link Archer and my home wifi. This means that all the traffic on the network now goes in and out of a VPN connection. Three is no longer doing any of the DNS or traffic routing/lookups.
Since I have put this VPN in the middle my internet has been absolutely lightening fast and not got stuck once! On any device.
So... here is a way for you to test if this solution might work for you.
1. Get connected as usual
2. Install Express VPN as a trial on your machine (other VPNs may do the same I expect)
3. See how your internet is on THAT specific machine while connected
If you find that this resolves the issue for long enough, you might then consider putting your own ExpressVPN router VPN in-between three and your home wifi.
Right now I have
WRT3200ACM with express vpn firmware installed
ORBI home wifi to get the signal through the house
Yeah, it's not a cheap setup, but now out in the country I have 50mbs flawless. Very happy.
There are very likely cheaper ways to achieve this but hopefully this triggers some ideas.
Finally, I did try setting up express VPN on the threeUK supplied 4g modem as it has space to do so. At the time I found that broke some things / it wasn't great so I removed those settings in the end. That may be worth exploring as perhaps with another VPN or your specific network setup that will work just fine.
Anyway, I hope this maybe helps someone