Based on the immigration law, you need to apply for the SB-1 visa to come backl to reinstate your green card. See:
https://travel.state.gov/content/travel/en/us-visas/immigrate/returning-resident.html
https://china.usembassy-china.org.cn/visas/immigrant-visas/returning-resident-visa/
Because of Covid 19, the US Consulate in Guangzhou has freezed such a kind of visas applicaiton now. So, you may have to wait for visa reopening (at least to March 31, 2021).
