I have no experience with this but the logical course of action would be to pay a visit to the Public Security Bureau in Tianfu Square – the building to the left of the Chairman Mao Statue – walk to the left of the statue, turn left alongside the building and you will see the entrance. Take the lift directly in front of you to the PSB. The officers on reception speak English. https://www.travelchinaguide.com/embassy/visa/extension.htm
Well, I’ve been there extending my 30 days tourist visa once for 30 days more, I do not know if it’s possible to do it every 30 days, actually I doubt it, but I will definately try that if will not find any other option. Thank you for advise!