If you have a project or case need a smart dual sim 4g router, i recommend a model H750 which from E-Lins Technology Co., Limited to you.

The E-Lins H750 series dual sim 4g router is a compact, ruggedized 3G/4G/LTE and Dual SIM cellular networking solution designed for mission-critical connectivity in the most challenging environments. It is qualified for M2M (Machine to Machine), IoT (Internet of Things) and In-Vehicle applications.
 

With an extensive list of safety and hardening certifications, the H750 dual sim 4g router is engineered to protect against extreme temperatures, humidity, shocks, vibrations, dust, reverse polarity, and transient voltage, which is widely used in more than twenty industrial fields, such as CCTV security surveillance, vehicle, Telemetry, vending machines, power control, AMR, traffic, oil field, weather forecast, environmental protection, street lamp control, post, bank and many other areas.

Multi-carrier 2G/3G/4G LTE support with Dual SIM card slot
Cellular/WAN RJ45/WiFi client failover and Load Balance (Bandwidth link bonding)
Supports LTE Advanced with SIM-based auto-carrier selection
Cloud-managed (with NMS network management system), TR-069, Web management, SMS, SSH/Telnet/Command, SNMP
WiFi (802.11 a/b/g/n, 2.4Ghz)
Certified 3G/4G/LTE enterprise/industrial grade internal modems
Built-in transient and reverse polarity voltage protection
5-40V DC voltage input range (5-60V DC option), Dual Power Inputs / Power Failover;
POE (Power over Ethernet)
GPS / GNSS support
Serial Port
DI/DO port * 4
TF (MicroSD card slot)
Supports Ethernet (T1, DSL, Cable, MetroE), WiFi as WAN, and Metro WiFi
Ruggedized to ensure always-on connectivity
Create real-time alerts to monitor uptime
Offer secure, guest WiFi to passengers
Advanced security, VPN, and stateful firewall to protect sensitive data
Robust Metal Case is ruggedized for vibration, shock, provide IP30 protection
Desktop, Wall-mount and Din-rail mount of installation

To further enhance the wireless M2M and IoT reliability, the H750 dual sim 4g router load balancing has a second SIM Card slot so that you can insert two SIM cards to get different applications.

1) Failover

With this using, usually you can get two SIM Cards from different mobile network. One is the primary SIM, the other is backup SIM. In the event that the primary network becomes unavailable then the router will switch to the backup SIM card and use the alternative service provider.

2) Load Balancing and Link Bonding

Being a 4G Load Balancing Router, the H750 dual sim 4g router load balancing can utilise the two embedded 4G modules, the RJ45 WAN and WiFi Client connections all at the same time and share these WAN connections with the LAN users.

With channel link bonding, the H750 dual sim 4g router load balancing makes two or more links are combined for redundancy or increased throughput. Channel bonding is differentiated from load balancing in that load balancing divides traffic between network interfaces on per network socket (OSI model layer 4) basis, while channel bonding implies a division of traffic between physical interfaces at a lower level, either per packet (OSI model Layer 3) or a data link (OSI model Layer 2) basis.