With the little information you provide, it could be several things. It would even be useful to know if there was no such problem before and it connected well and “something” happened in the time frame when it started to fail. “Something” like an update, the node shutting down incorrectly, etc.
For example, your router may have DHCP disabled and the node does not obtain an IP automatically. This is easy to find out if you have other devices on your network that use DHCP.
Try discarding DHCP and assigning a static IP to your node. This way, if it does not connect, then the problem is bigger and comes from somewhere else. The umbrel log can help us detect the problem.
If you want to try the static IP, you can follow these instructions.