Can't connect to Prometheus datasource, but can connect to node-exporter

Hello, newb here, I have just downloaded and installed:
Prometheus 2.48.0

Grafana and Prometheus are running on a Raspberry Pi 4B,
Linux raspberrypi 5.10.103-v7l+ #1529 SMP Tue Mar 8 12:24:00 GMT 2022 armv7l

I have node-exporter installed on a second Raspberry Pi.

I want to create a dashboard showing resource utilisation for both Raspberry Pis, to begin with.

When I edit a visualisation/panel in the dashboard I can only select the second Pi as an instance to a query. The data source is set to “Prometheus (default)” for the panel. I tried changing the instance manually to localhost:9090 and also the first Pi by IP address, i.e. [ip address]:9090 but it doesn’t appear to be recognised.

If I go to the Grafana Administration/Data sources menu I can see that
http://localhost:9090 is set as the default data source

I don’t see any errors other than I cannot select the Prometheus server as a data source.

If I point a web browser at http://[ip address]:9090 I can access the Prometheus server and see the two Pis as targets,

Thanks in advance.