I’ll be honest, I have a SH3 and a mesh network (Orbi). I have my NAS connected to the SH3 along with the main Orbi node. Both of them do DHCP – although on the SH3, I’ve got fixed addresses for the NAS and the are on different subnets (192.168.0 for the SH3 and 192.168.1 for the Orbi). The SH3 is NOT in modem mode, but I have turned off the Wi-Fi network (after migrating everything to the Orbi subnet/Wi-Fi). It may not be pretty, but it works. I know that ideally everything should hang off the Orbi, but history and physical location of the SH3, Orbi and NAS means that it i snot practical!
Still get the max speed on the Virgin BB (even from Wi-Fi connected devices on the Orbi subnet) so I know it works!