You need your phone to enter the hard reset command for your Stryd. If your phone cannot connect, you cannot enter the command.
The workaround is to fully drain the battery of your Stryd. If your watch can still connect to Stryd, fully charge the watch and start recording a run. This will put Stryd into run mode (if you have a Garmin watch) and the battery will drain in maximum 24 hours. Once the battery is fully drained, Stryd will do a hard reset when placed on the charger.
Fully charge your Stryd and your device is reset.