This is what Layer 3 is for. You need to open the relevant port between vlans (e.g. TCP 443 for https) on the firewall. I think its UDP 1900 but may vary by appliance.
I’d also allow multicast, ICMP (ping) and DNS between your vlans as a minimum depending on what they’re used for.
I used to use all the Simple tools, even paid for them, unfortunately the dev changed monetization to a shitty model and I got locked out.
Since then I moved to QKSMS. Its on F-Droid and despite some initial quirks I’m very happy with it.
I’m still bitter about them removing the alarm function from the BBC radio sounds app
Late response but I got to the root of the issue. Lemmy (well, lemmy.world) wasn’t being cached by Quad9’s DNS servers.
I tested by using different DNS servers (OpenDNS) which worked fine, then permanently resolved it by installing unbound on my pihole and using that instead.
Tl;dr it was DNS
It’s always DNS
Have you ever seen the opening credits to Duck Tales?
That.