An LTE Category 6 modem can bond channels together, so if you are using Three or EE and have both Band 3 (1800mhz) and Band 20 (800mhz) signals available, you can achieve faster speeds.
Generally if you are distant from the transmitting mast, you need external antenna and the shortest cables to the modem you can achieve.
Phones are category 20 or higher now, including expensive mobile hotspot ("MiFi") type devices from Netgear, warning expensive
Some of these have external antenna connections, such as my 2.5 year old Netgear aircard 810.
80/20 (2/jun/14) at 470m - Sync history highest: 64/9 (Sep/17), 54/6 (Jan/19), 51/6 (Mar/19)
20 years of broadband from 1999's ntl:cable modem trial - Live BQM