I have seen recommended the GL·INet Flint 2 GL-MT6000 openwrt.org/toh/gl.inet/gl-mt6000

I’ve used a Flint 2 GL-MT6000. Works fine for me. If you want a high-spec new OpenWRT machine, it’s near the top end, from my skim. It may be overkill if you don’t intend to do anything elaborate.

and is also failing a lot lately (it loses internet connection at least once a week and needs restarting).

So, you’re still going to need to have a cable modem or DSL modem or whatever, if you have cable or DSL service, and it (or some other hardware, if you can get that working) going to be necessary for that bit. There are three pieces to this equation: a modem or whatever translates your satellite/long-range-wireless/cable/DSL/etc link to your ISP to Ethernet. A wireless access point, to provide WiFi. A router, to NAT connections, do things like setting up what traffic goes where, what’s blocked, etc. The Flint 2 will act as a wireless access point and as a router. But it doesn’t have a modem built in.

You can turn off a lot of functionality on your ISP-provided router, but you’re going to keep it in the loop (with the Flint 2 plugged into it) unless you have the ability to use an alternative modem. It may be possible to replace it with a different modem, depending upon your particular situation; I believe that some ISPs may embed authentication information in the modem specifically to keep people from using alternatives.

Thus, if the ISP-provided router keeps failing in that configuration, this isn’t going to fix your problem. You can reduce the functionality that the thing uses, like, maybe throw it into bridge mode (which just passes traffic through, doesn’t NAT anything) and let the Flint 2 act as a router. That’ll reduce what the ISP-provided router does, and if that’s triggering some bug on the thing, that might resolve your problem.

If the existing router is honestly failing in some way—like, once it has problems, it never comes back without a reboot, isn’t just connectivity problems—you might request a replacement.

If none of the above fixes your problem (i.e. the ISP-provided router, stays in the loop and keeps failing and only a reboot fixes things) a mitigation, albeit not a fix, would be to set something up on your network to ping the outside world and then power-cycle the router automatically if it fails for a certain amount of time.