I am experiencing a similar conundrum...my 'dial-up' internet connection appears to be about three times faster than my mobile modem broadband connection. Both with Virgin. ( browsing PCAdvisor on both machines) when I try to display The 'dongle' tariff ie what I've got left on my monthly allowance, I get the message 'not available, try again later'. Both computers have similar specs/overheads etc.
OnePlus 5T review: Hands-on