Poor dongle bandwidth - 20Mbps max down?

Hi, I've been with Smarty for a year or so and primarily use the SIM in a Huawei dongle to provide failover connectivity to my network if my main ISP goes down. 

I've only ever been able to get a max 20Mbps download. I can get up to 50Mbps upload, but maximum down seems to be 20Mbps. The download speed means that I am connected on 4G as 3G is less than 3Mbps down. 

According to the Ofcom signal strength prediction tool, I should have good signal strength indoors as well as outdoors. I live almost on top of a cellular macrocell which appears to host all networks. 

If I take the SIM and dongle elsewhere in the UK if I can get a signal, I have never seen more than 20Mbps. I've checked the dongle configuration (no user or password, just auto authentication and mob.asm.net as the APN). 

I don't understand what is going wrong here. I don't understand why I should see 12.5Mbps down and 48Mbps up for example.  If I use a work Vodafone 4G dongle I get 108Mbps down and 40Mbps up pretty much consistently. I get that it is a different network, but proves the hardware that I am using the dongles with. 

Any thoughts very much appreciated :) 

Thanks