Comment by sam_goody
I use uptimekuma (self hosted) and am very happy - it can text, or just about anything else - as long as you set it up.
Also Tianji[1], self-hosted in the same space.
Of course, these require setup and have to be hosted somewhere. Either another server or deployed to Heroku (Sealos, Render, etc), since they obviously cannot be deployed on the server you wish to monitor!