Hello @HelloWill,
Trade show are always challenging locations with any form of wireless communications, you often are within an RF shielded building (be it due to metal roofs and walls and concrete), there may be a DAS operating by one or multiple of the carriers (a Distributed Antenna System), though during trade shows and conference the area becomes saturated with mobile devices (phone, WAPs, two way radios, and many other RF devices).
If you have already done what @jmjones mentioned and also what @MartinLangmaid recomends, then you may find the following explains some of what you are experiencing (this based from experiences at many exhibitions and conferences).
Previously we use to regularly parked an RF scanner looking at activity in an area of to one side at conferences and exhibitions. An example of the results at a recent broadcasting exhibition in Sydney over the duration of just 30 minutes we picked up over 100 people moving around operating with there built in Wi-Fi from there phones, plus there was the exhibition sites Wi-Fi, several of the exhibitors doing there own Wi-Fi and also Wi-Fi detectable from the exhibition next door. Now imagine the load on the mobile/cellular network when so many people and exhibitors all in the area.
We too provide services to exhibitors, though we always push for them to go with a Wired Connection from the venue (yes we know the venues charge like wounded bulls for the privilege), though it gives the best guarantee of service normally. We avoid like the plague using any USB dongles at exhibitions/conferences to provide services, we always use an industrial modem with a MIMO solution antenna properly installed.
Once you have a MIMO option and a venue wired connection, Peplink routers then become an excellent option to manage the connection with some potential redundancy through the mobile/cellular network options or with another wireless service (using the existing features already with the Peplink MAX range).
Based on the hardware you have, being the MAX 700, we would suggest (in this order):
- Get a physical connection at the venue as your primary connection (make sure it is not a wireless relayed connection) and use that on the WAN connection,
- The MAX 700 has WI-FI WAN available, use that to connect to a venue supplied Wi-Fi service (look for 5Ghz service, 2.4Gz will become very congested during the exhibition/conference), this is then your second WAN connection,
- Keep your dongles as a fall back or put them on a lower priority the options 1 and 2.
Finally, some carriers enforce an automatic load-balancing and network priority on there networks, although you may be only getting 20Mb on your USB Dongle compared to 40Mb on your phone, the APN used in the USB Dongle combined with the manufactures IMIE may result in the connection having a higher priority within there carrier network network and higher levels of service guarantee, you need to ensure that the assigned SIM cards APN you are comparing to on the Phone and the Broadband Modem (USB Dongle) are identical and on the same account (this is more than just moving the SIM card between devices).
Remember that cheep USB dongles do not always do all of the available frequencies you will find in a modern phone that carrier may use and not all dongle are capable of operating across multiple channels simultaneously.
Wireless communications is full of “gotchas” like above. See how you go with all of the above advice and information.
Happy to Help,
Marcus 
