How do I stop a small WordPress website crashing so regularly on Digital Ocean account?

Sorry, not sure how or where to ask this, but this seems a better place than SO.

I have a small WP website hosted on a Digital Ocean droplet and it keeps falling over with an “Error establishing a database connection” error. Restarting the server resolves it. This is a bog standard, (very) low traffic WordPress site.

I’ve hosted similar, small WP sites for many years on standard shared hosting but for some reason, now I’ve moved this site to DO, it falls over on an absurdly regular basis. I’ve checked the plugins, security and such and can’t see anything that would cause this. I’ve upgraded the account three times but I was under the impression a small site like this could be hosted for a couple dollars, not the $25 or such I’m paying now.

Again, this is a very low-traffic, bog standard WP website and I can’t see anything out of the ordinary considering I’ve had experience with WP sites for over 10 years and I didn’t have any issues before moving the site. Other than a one page portfolio, this is the only site on my account,

I feel like I’m missing something obvious, and as someone who’s not very good with DevOps, I realise I’ve made a mistake choosing DO, but I’m hosting this site for my sister and I’m hoping there’s some sort of obvious thing I’m missing before I move it again.