Honestly if that's the only spot where there's an issue I'd get an old iPad LTE and solar panel and permanently mount it somewhere with solar. Then the airtag will be able to find your doggo there as well.
It's cheaper than fencing thousands of feet of perimeter, anyway.
That creek is miles long, forms one border of my property, and its all of below grade. I'd have to install numerous repeater stations to accomplish that, all on somebody else's property, all along that creekbed. Turns out the government and the telecos have made those investments for me.
It's cheaper than fencing thousands of feet of perimeter, anyway.