Can't connect with IDMobile - but can with Three and Vodapho

I'd like to understand what the cause as to why I cannot connect to:
- Santander website
- premium bonds website (nsandi.com)
- my company VPN

Hardware: Huawei B535 router

Details.
When I use IDMobile SIM in the router, I get connected using 4G+ (LTE) with speeds ~50Mbs. However, I cannot reliably connect to the above sites The NS&I and Santander sites tell me that I've been logged out for security reasons. When I try my company VPN, I cannot get to the 2nd stage of authentication, my connection is just dropped

However, if I swap the SIM to be a Three SIM (Same underlying network??) - everything works just fine. (but it rarely connects using LTE speeds- just plain 4g (10Mb/s)). When it does connect using LTE, it works.
If I swap the SIM to Vodafone, then I get 4G, and it works just fine.

What's likely to be going on? Helpdesk man from Santander said that my "IP address was changing too frequently when using the website from my desktop, and so I'm dropped". He suggested I use the mobile phone app (via my phone - also on IDMobile) - and that works fine.

(I went to mobile connection, and disconnected my landline, as I was only getting 8Mb/s from BT or Sky in our area )
Can anyone explain to me what is going on here?

Possibly a CGNAT issue

In reply to a post by gandalfmeister:

Can anyone explain to me what is going on here?

4G & 4G+ are both LTE. In a road analogy the 4G+ is indicating that more lanes are available (motorway), but if there are a lot of cars you can still be stuck going slowly, e.g. M25.

All mobile networks (and Three's services for phone handsets) essentially have a router doing "sharing" in the network (known as CGNAT), then you add your B535 which does this in your home as a traditional router. If you had traditional broadband this would be the same as using two routers one connected to the other.

This can stop many VPNs working, but the websites should work, if they work in a mobile browser, however it is likely something their security teams are doing for additional protection of money.

The VPN not working will depend on the type and technology in use that your company has chosen.

If you use the Three SIM and use their mobile data APN then you can avoid CGNAT, but there is no guarantee the IP doesn't change. With all the other real networks then you can't avoid CGNAT.

ID Mobile is a virtual network run by carphone warehouse and uses the Three infrastructure. It is intended for phones and may not offer the avoiding of CGNAT option.