Newbie to OpenWRT, but fairly Linux/network fluent. Set up DHCP to start at 50 with limit 250 (my bad, thought it was a start-end, not a start-length). There is a bug hiding there somewhere. Auto-revert didn't work. The router continued functioning but could not login to it (web or ssh). In the end, used the debricking power option https://openwrt.org/toh/linksys/linksys_wrt3200acm#power_switch to restart with stock firmware, and then continued from backup.
A series console is always a good idea when getting into this kind of thing. For the device you linked, it looks like it's internal rather than easily accessible, but might be worth taking a look for the future if you are going to play with OpenWrt at anything but the flash and forget level (and even then....)
For posterity: A misconfigured DHCP server can never "brick" a router (besides "bricked" being the wrong term here anyway.)
Without any deeper insight into the concrete network setup, I would guess that you lucked out and DHCP assigned your client one of the IPs that spilled over into the next network segment.
E.g., if your LAN is at the default 192.168.1.1/24, and you erroneously set DHCP to start at .50 with limit 250, 45* of those IPs would spill over into the next range, i.e. into 192.168.2.1~192.168.2.45, which the rest of the router's configuration does not recognize as LAN. Reconnecting wouldn't help either because DHCP assigns IP addresses pseudorandomly based on the client's MAC address, would still be assigned the same one every time.
*) don't quote me on that exact number, there might be one or more off-by-one errors in there, but you get the point
However, I don't see why you couldn't still manually set your client to an IP in the router's LAN range. Following the above example, if you manually set 192.168.1.2 for your client, you would be able to connect again and correct your mistake.
You mean the spillover? Nope. Granted, the setting is a bit clunky, but that's intended behaviour, some people want/need to use a larger range than /24, and DHCP to give out IPs across a larger range too.