I used the whitelisting suggested by Glitch
<span style=“color: #222222; font-family: source_sans_proregular, ‘Helvetica Neue’, Arial, Helvetica, Geneva, sans-serif; font-size: 14.6667px; background-color: #f9f9f9;”>Luci > network > DHCP and DNS > general settings.</span><br style=“outline: none; vertical-align: baseline; background: #f9f9f9; margin: 0px; padding: 0px; color: #222222; font-family: source_sans_proregular, ‘Helvetica Neue’, Arial, Helvetica, Geneva, sans-serif; font-size: 14.6667px;” /><span style=“color: #222222; font-family: source_sans_proregular, ‘Helvetica Neue’, Arial, Helvetica, Geneva, sans-serif; font-size: 14.6667px; background-color: #f9f9f9;”>The last box is “Domain whitelist”. Enter the captive portal domain here eg. captiveportal.com.</span>
Go to your browser and either try the captive portal address or in my case I just refreshed a page. Don’t try a SSL site though you’ll get a warning because it will have an invalid certificate.