Lightsale instances cannot connect to Lightsail database

I have a web app running on Lightsail instances stacked with LAMP (PHP 7/MariaDB 10 service). When I want to connect the instances to a Lightsail database instead of localhost, it works sporadically. The front end reads "cannot connect to MySQL database" and in the database server log I read : [MY-010055] [Server] IP address 'xxx.xxx.xxx.xxx' could not be resolved: Name or service not known" where the IP is the private IP address of my instance.

My instances and database are in the same region. Even though it is not necessary I have tried enabling VPC peering and opening the db to the public.

I can connect to the database using the command line tool: mysql -u uuuu -p -h hhhhh (ie: it is not a problem with entering the wrong credentials)

The web app seems to work with the dedicated Lightsail db server when I am the only one using it and until I open it to more traffic, then I get the connection error.

I have increased the number of max_connections from 65 to 500 using the aws CLI (localhost is 150). My current traffic is 2000 visitors per day.

aws lightsail update-relational-database-parameters --relational-database-name Database-1 --parameters "parameterName=max_connections,parameterValue=150,applyMethod=immediate"

Localhost is Mariadb 10, dedicated database server is MySQL 8

SSL is handled by Cloudflare. There's no SSL on my instance.

I don't want to keep using localhost dbs, I want to centralize my data in a dedicated database.

Does anyone know how to handle this problem?