MACVLAN is not a replacement for VLAN, use VLAN whenever possible.
Some modems, especially ISP-supplied ones, may cause rogue DHCP scenarios. They reject DHCP messages, but the server remains on for internal reasons. MACVLAN is not suitable in this case.
MACVLAN is used for budget setups.
Do not enable software offload in the firewall when using MACVLAN, as it may affect the ARP table.
Some white-label brand 100-Mbit switches may perform poorly with multiple addresses from the same port due to limited switching fabric. Use a cheap gigabit switch.
Disable IPv6 on all modems.
Diagnostics
Simple Check
Microsoft Rogue DHCP Checker should show one DHCP server:
Advanced Check
For Linux/MacOS: Disconnect SmoothWAN and the client from the network. Use Wireshark to listen on the broadcast, reconnect the client to the network, and observe any DHCP messages. There should only be a single source.