Hello ! Is it possible to set up Rancher to use Let's Encrypt as a Certificate Issuer using DNS challenge for authentication?

You can use a certificate management tool like cert-manager to setup an issuer using a DNS challenge, then configure your Rancher instance to use the secret created by the issuer.

I'm not too familiar with OVH, however, I believe you can create an issuer by creating a webhook provider. There is this OVH webhook for cert-manager that you should be able to use to create the issuer.

I am uncertain whether I should use the installed cert-manager webhook or the one you recommended, since the recommended webhook does not have a Helm chart installation method available

That decision is really up to you, although there is a Helm chart within the repo I mentioned, and there are some steps to install the chart with your unique group name.

Sorry if my previous message was not clear enough, but I couldn't find a Helm repo to install the chart from

Ah okay, I see what you mean. What you can do is create a repository within Rancher, targeting that Git repo specifically, then you will be able to select the chart from Apps -> Charts page.