Minimum firmware: 18.4.54

Create a new custom script under System -> Scheduled tasks -> custom scripts, and enter in the following.   Adjust the Interval to 20m, or the desired interval you would like this script to run.

#!/bin/sh

# Check the signal strength of the modem.  If it's below the specified percentage,
# mark it as a failure.  Keep a count of failed signal strength thresholds. If
# the signal is below the threshold for four consecutive checks, then reset the modem.

# Adjustable settings
signal_threshold_percentage='20'
fail_count_file='/tmp/custom_signal_test_fail_count.txt'
fail_count_limit='4'  # number of concurrent failures that occur before resetting modem

test_failed() {
  try=$((try+1))
  echo "$try" > "$fail_count_file"
  accns_log w config "custom: signal below threshold ($1 - try $try)"
}

test_passed() {
  rm -f "$fail_count_file"
  try=0
  # Note: uncomment the following line if you want to log successful tests
  #accns_log w config "custom: signal above threshold ($1)"
}

try=$(cat "$fail_count_file" 2> /dev/null)
try=${try:-0}

# make sure $try is an integer. set to zero if not
case $try in
  ''|*[!0-9]*)
    try=0
    ;;
esac

# do the signal strength test
current_signal_pct="$(runt get mm.modem.$(modem idx).status.per)"
current_signal_pct=${current_signal_pct:-0}
if [ "$current_signal_pct" -ge "$signal_threshold_percentage" ]; then
  test_passed "current=$current_signal_pct~threshold=$signal_threshold_percentage"
else
  test_failed "current=$current_signal_pct~threshold=$signal_threshold_percentage"
fi

# reset modem if failed count is greater than specified limit
if [ "$try" -ge "$fail_count_limit" ]; then
  # note that we reset the fail count, to give the modem time to reconnect and
  # improve signal before we reset it again
  rm -f "$fail_count_file"
  modem reset
fi