I have the same problem. permanently!
I have about 50 pcs. RUT955 routers in use. The devices are in use all over the country and I have my own APN at the providers LTE network. I can access all the devices through this VPN network. Every device has the same configuration. The web access (https) and the ssh access are enabled. The ssh connection can be established every time, BUT the web interface only works in the first few days. After some days the interface is not accessible.
When I enter the URL in the Browser (Chrome, Firefox, Edge), at first I get the page with the (standard-)information that the certificate is not valid (NET::ERR_CERT_AUTHORITY_INVALID). When I click the continue button the browser is loading, loading, loading, .... I get no UI.
When I log in to the router via ssh and reboot the web server (/etc/init.d/uhttpd stop and /etc/init.d/uhttpd start) the web interface starts working. With this bug the web interface is unusable.
Any Idea what I can do? Is there a possibility to restart the web server every day (workaround)?