To troubleshoot, try connecting a device (either your home security panel or a PC) directly to the Zyxel modem and check if you have Internet.
I had the same issue happen to me yesterday with my Surf SOHO MK3. In my case, the problem lies with the Surf not with the modem. I had to restart the Surf 3-4 times before it would reconnect to the modem. In 11 months of use, I have never had a problem until yesterday. And I wasn’t changing any settings/cables/etc.
This issue seems to be similar to the one that’s affecting users of the AP One / AC Mini / Balance 20 described here:
Also while troubleshooting this, I noticed that I am now only getting 20-25 Mbps download speed when connected to the Surf via WiFi. I used to get the full 30 Mbps (still over WiFi), the last time I checked. My WiFi connection to the Surf is 144.5 Mbps so that isn’t the bottleneck. When I connect to the Surf via an Ethernet cable, I get the full 30 Mbps download speed.
Is anyone else having these issues?