I downloaded everything from (https://github.com/plewin/tp-link-modem-router) to ~/bin on my Raspberry Pi. There's a lot of files and I'm sure most of them can be discarded for just sending SMS messages.
The script to send an SMS is simple
#! /bin/bash
LOGIN=$1
PASSWORD=$2
RECIPIENT=$3
MESSAGE=$4
ROUTER=http://192.168.1.1
cd ~/bin/tp-link-modem-router
./sms-send.js --url="$ROUTER" --login="$LOGIN" --password="$PASSWORD" "$RECIPIENT" "$MESSAGE"
And in Node-red
[{"id":"aff00f528fa10668","type":"group","z":"820b503366768e45","name":"Send SMS","style":{"label":true,"stroke":"#0070c0"},"nodes":["13fc9fccc6a88453","213afa80544096af","d2cb636eb4901b93","ac649fbea5eed8da","7c296b0377c4345c","fc2dd2efe4ca4601","acbe2007c8ab515c"],"x":14,"y":19,"w":852,"h":202},{"id":"13fc9fccc6a88453","type":"link in","z":"820b503366768e45","g":"aff00f528fa10668","name":"Send SMS","links":["b75d8b3dc668ef41","a558b7f73db3ca09","3ab9de214b5bd153","5cfd364c8c5c7a45","334bad91d6df2df9","d1ad28d60ebc7b40","785923d87b307e3f","4fb14051335559ee","800dfba849475560","94114d6ac3d3a97e","db554a5e3a178bfc","033bbc952bc1c24d","5a4ad1189403783b","ea62ef5701b5fe46","a0f906f30a2baf0a","e21a82870655932b","e58705dc3051e752","e142daf455487cfd","7c183baa42443b43","212787a022d1f3d3","1f0b8de909826aa4","c9c025271347fe21","de857cbaf7595198","ce92e710d4b3872b","32e2c61b8d0a5eaa"],"x":155,"y":120,"wires":[["d2cb636eb4901b93"]]},{"id":"213afa80544096af","type":"exec","z":"820b503366768e45","g":"aff00f528fa10668","command":"/home/pi/bin/sms.sh","addpay":"payload","append":"","useSpawn":"false","timer":"","winHide":false,"oldrc":false,"name":"","x":440,"y":120,"wires":[[],[],["acbe2007c8ab515c"]]},{"id":"d2cb636eb4901b93","type":"function","z":"820b503366768e45","g":"aff00f528fa10668","name":"","func":"const routerlogin = global.get('routerlogin');\nconst routerpassword = global.get('routerpassword')\nconst recipient = global.get('smsrecipient')\nmsg.payload = routerlogin + \" \" + routerpassword + \" \" + recipient + \" '\" + msg.payload + \"'\" ;\n\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":260,"y":120,"wires":[["213afa80544096af"]]},{"id":"ac649fbea5eed8da","type":"inject","z":"820b503366768e45","g":"aff00f528fa10668","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"A Test Message","payloadType":"str","x":140,"y":180,"wires":[["d2cb636eb4901b93"]]},{"id":"7c296b0377c4345c","type":"comment","z":"820b503366768e45","g":"aff00f528fa10668","name":"Send SMS from tp-link mr600","info":"","x":160,"y":60,"wires":[]},{"id":"fc2dd2efe4ca4601","type":"debug","z":"820b503366768e45","g":"aff00f528fa10668","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":750,"y":120,"wires":[]},{"id":"acbe2007c8ab515c","type":"change","z":"820b503366768e45","g":"aff00f528fa10668","name":"Result","rules":[{"t":"change","p":"payload.code","pt":"msg","from":"0","fromt":"num","to":"SMS Message sent","tot":"str"},{"t":"change","p":"payload.code","pt":"msg","from":"1","fromt":"str","to":"Error sendng SMS","tot":"str"},{"t":"set","p":"payload","pt":"msg","to":"payload.code","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":610,"y":120,"wires":[["fc2dd2efe4ca4601"]]}]