Why have 2 routers? Ditch the ISP one, and just use your “good” one.

Try "forgetting *the network.

https://support.my.uq.edu.au/app/answers/detail/a_id/2509/~/how-do-i-remove-or-forget-a-wifi-network%3F

We use NSM devices extensively throughout the Far North Queensland Wet Tropics Rainforest, with monsoonal rains, 38c plus temperatures with high humidity and they don’t miss a beat.

The biggest problem we have is that the trees grow in to their transmission path and need pruning.