That's a great question. Is your MR1100 unlocked? If it's the AT&T version, you'll need to request an unlock code from AT&T.
Regarding Verizon, it should theoretically work, but the performance might vary. Although AT&T and Verizon utilize many of the same LTE radio bands, there are differences in their configurations (sub blocks) and carrier aggregation (CA) combinations.
I've observed that my MR1100 can detect both Verizon and T-Mobile networks when I switch from Auto to manual network scan. However, I haven't tested it with a Verizon SIM card, although I do have a T-Mobile SIM. Admittedly, I haven't tried it yet because I have unlimited data with AT&T.
You'll also need to adjust the Access Point Name (APN) settings from the default AT&T to the Verizon APN settings. There are some reports suggesting that the MR1100 might have a bug where it doesn't retain "non-standard" APNs (i.e., those not from the primary carrier). However, I can't confirm this. If it's true, you might need to re-enter the APN settings each time you restart the hotspot.
The Netgear Nighthawk M1 is compatible with Verizon's network in terms of hardware capabilities, as it supports many of the same LTE bands that Verizon uses. However, compatibility issues may arise due to differences in network configurations and carrier-specific settings. Verizon often has specific requirements for devices to work optimally on their network, including certified firmware and provisioning settings.
If the Nighthawk M1 is not officially certified by Verizon or lacks the necessary firmware and provisioning, it may not work as well or at all on their network. Additionally, Verizon may prioritize their own branded devices or have restrictions on devices brought from other carriers. Therefore, while the Nighthawk M1 can technically connect to Verizon's network, it may not offer the same level of performance or reliability as devices officially supported by Verizon.
Please get a Verizon compatible device that you are sure will work with no issues. providers of stable networks can provide some good solutions.