There’s not much we can do to help you Jas our end, however, there’s a couple of suggestions within the Thread from members with a bit more IT knowledge than most of us have.
I have BT fibre infinite thing but always access this
On the Tapatalk app on my Phone and have no issues so when I get home I’ll try and log in on my
Laptop just to see and send Some Feedback in here
There's nothing wrong with the certificate, still a while before it needs renewing. I use Lets Encrypt on several of my servers.
If you're having problems then I suggest you take a look at your firewalls; either on the device you are accessing from or from the router (home or office), which may be configured too aggressively. BT like many broadband providers offers you the 'option' of configuring a family-protect firewall (Web Protect) on their Home Hub routers, but you don't have to use it..