So, time for an update....
Bought a second hand Draytek router (2830n) which has a USB port that allows a 4G dongle to be used as modem.
Bought a data sim card from Three with a few GB of data on.
Plugged it all in. No joy. I might be using the wrong settings for the modem, but i don't think so. It looks like the USB modem i bought is not compatible with the router. According to Draytek (
https://www.draytek.co.uk/download/support/DrayTek_USB_Modem_Support_160117.pdf) the modem is compatible with the later version of my router, but not the version I've got.
I tested the usb 4G modem just plugged straight into the PC and i get pretty good reception and 20 Mbps download and 10 Mbps upload. A massive improvement on our ADSL.
So i think we're going to go ahead with using LTE for our home internet service and scrap the ADSL altogether
I now need to find the right hardware to get the best out of it.
Anyone got suggestions for a LTE router. Or I could use an LTE modem + the Draytek.
Not sure whether its worth putting an LTE antenna on the roof to maximise reception.