Have to disagree on that.
Foolish, false sense of security: It is as much secure as secure your firewall is.
Disable IPv6. Block all incoming ports TCP, UDP, ICMP. For the last one you may have an option "Disable WAN ping", it is the same. It can be more firewall options for ICMP, the only right one is 'drop'. It creates a blackhole for all unexpected incoming traffic. The other option is like answering "No, I am not here" to the question "Are you here?", quite foolish if you really want to hide.
Disable UPnP on the WAN interface, it is important, people forget about. You can change firewall to the more strict type (like cone vs. symmetric on Huawei LTE routers). Symmetric is more strict, you will have to manually setup trigger rules. It doesn't brings much improvements in security and there is more work to do, so maybe forget it.
If you open some incoming ports, additional meassure is adviced, see post above. By setting up firewall to respond only to the specific incoming IP address you will ignore all unexpected visitors. If it applies to your environment, you are very safe. If it is not possible, use non-standard ports. Do not reply to this port directly, but trigger opening another port and verify protocol, request type before forwarding packets to the LAN.
Verify that your router is not on the list of unpatched devices for known backdoors.
