I run https://www.waveform.com and we help a lot of folks like yourself get better signal. It's not really worth messing around with cheap options - your productivity is on the line and it's worth spending a little for great connectivity.

With that in mind, the best option is a Pepwave MAX HD2 Dual SIM. Then put a Visible SIM from Verizon in one slot and a Calyx SIM (T-Mobile) in the other, or sign up for the AT&T iPad plan. Either use whip antennas or ideally use a roof-mount 4x4 MIMO antenna to get the best signal.

Using Pepwave's VPN you can run WAN smoothing and/or bonding to make sure that your connection is rock solid.

Worth looking the LTE hacks facebook group, though they tend to use cheap gear, and I'd recommend using something beefier.