I'm using a 4G Router (TL-MR6400 with EE data SIM) because we are 5km from the nearest exchange and speeds are worse than dial-up!

I'm getting Secure Connection Failed when visiting some sites, most of the time. Sites like YouTube or Facebook connect sometimes, but most of the time I get this message. I'm using Firefox, but it's the same with Chrome/Edge. If I swap to tethering off my phone (S8), everything works fine. I'm assuming it's a setting within the router, but I haven't located anything going through some of the TP help.

Thanks!

Phil