Is there any difference between Dual SIM and Dual Modem routers?

I was trying to find a router with dual cellular connectivity. My quick question is, what is the difference between Dual SIM and Dual Modem? Thanks!

Dual SIM routers have a single cellular modem with double slots to insert two SIMs. With these routers, you can connect two different carriers but have one active carrier at a time. When the active carrier goes down, the second carrier can be activated.

In North American cellular market, a modem must have a specific carrier firmware image (modem firmware) loaded to work reliably (ex: VZW, ATT, Generic (T-Mobile, Canada), Sprint) on the respective carrier network.

Some of the dual sim routers and their accessories are available here.

For dual modem, the cellular gateway will have two physical cellular modems available internally (they can be embedded, modular, or a combination of both), or with an option to extend an external modem e.g. cradlepoint dual modem capable routers.

Dual-modem functionality is an excellent way to attain seamless, instant carrier redundancy in your network. See more on link aggregation.

To improve signal reception, leverage cellular antennas that can be attached to these routers.

Because the gateway is a dual SIM (single modem) and not dual modems, it cannot provide two simultaneous connections or be load-balanced. Dual Modem: if you want to utilize multiple SIMs simultaneously for a faster connection/higher bandwidth (ex: Load Balancing or Bonding).