Apologies for getting on this issue so long after the OP.
I just swapped my HH5 for a Smart Hub 6 and I was getting intermittent connections to my Airport Express and Airport Time Capsule which are LAN connected. They seemed to disappear from the network when my MacBook Pro was connected on the LAN only.
Firmware SG4B1000B316 My WiFi is split and I have Smart Setup turned off.
I changed the IP4 DHCP lease time down from 1 day to 6 hours and this seems to have solved the issue for the moment.