Just tested RUT955 router with the latest firmware version RUT9XX_R_00.06.05.3 and HTTP POST/GET sms_send command worked just fine.
Router was able to send SMS when using both "00" and plus sign "%2B", when entering phone number.
In my tests I tried with multiple browsers (Chrome, Firewall, Edge) and in all cases I was able to get SMS messages from the router.
If you would go to Services -> SMS Utilities -> SMS Management -> Send SMS window in router's WebUI, you will be able to send SMS directly from router's WebUI. Please check whether you will be able to receive SMS from router if you would use "00" in phone number, when sending SMS directly from WebUI.
Also please check whether you will not be able to receive SMS using POST/GET string with different browsers.