How do I fix a robot vacuum that keeps losing Wi-Fi connection mid-cycle?

Use the 2.4GHz band. Robot vacuums have tiny, cheap Wi-Fi chips that barely handle 2.4GHz, so band steering with 5GHz confuses them. Split your router’s bands, give the 2.4GHz network a different name (e.g., “Home-2G”), and connect the vacuum to that. Also keep the charging dock within decent range of the router.