So, you have the iSolar app on your phone using wifi to connect to the wifi dongle on your Sungrow inverter. There may be interference caused by the setting on the TPG wifi router. Can you use your phone to check the signal strengths of the Sungrow and the TPG router.
On the TPG router, you can change the 2.4G band channel to 1 or 6 or 11 and see if the phone can connect to the Sungrow. Or, the Sungrow might have same ability to change channel.