My Backyard Connects Ancient and Modern

Broke miner uses backyard portal to feed ancient tribe, gains gold, then arms them with modern weapons against invaders.

A penniless miner uncovers a shimmering rift in his yard that opens onto an ancient civilization facing famine. He starts ferrying contemporary food supplies through the gateway, and the grateful tribe repays him with chests of gold that erase his debts. The arrangement shifts when a hostile army from that era marches on his new allies, forcing him to switch cargo from calories to firearms and explosives in a bid to defend the settlement.

• fantasy
• time travel
• action

1. Miner discovers backyard portal to ancient world
2. Trades modern food for tribe's gold
3. Uses modern weapons against ancient army

Anyone who likes a resourceful protagonist leveraging modern tech to upend an ancient conflict will appreciate this setup.