Thanks for getting back to me.
The plugin I wish to install is Seriously Simple (but it doesn’t matter what the plugin is, because according to my dashboard, I need a business plan to install any plugin).
On my www.remarpro.com website page, I’m looking at the My Sites tab, which has a load of menu items down the left hand side of the screen, one of which is “plugins”. When I choose that option, I get the following message: To install plugins you’ll need to upgrade to a Business Plan.