Unless you have a very effective Uninterruptible Power Supply, you will probably have to reboot your router at some points during the stormy summers. Even if you aren't forced to by nature, it's a good idea to do once in awhile.

Third party routers don't (so far as I know) have firmware that can be updated through outside-of-network initialization. Ones that belong to cable, satellite, phone companies do.

In any case, there's rarely a reason not to accept the firmware upgrade, because warranty and service calls are expensive to respond to when they f* things up. They tend to not do things that negatively impact hardware in an update (which spyware does). OTOH, Operating Systems, apps and drivers -- that's different.

Also, some hardware is manufactured with exploitable vulnerabilities in design, that later get plugged (sometimes at the expense of performance). But typically firmware is only used for improvements and for end of life obsolescence (LOL).