I filed my 2019 return by mail prior to July 15, 2020 after my e-filed return was rejected because AGI reported for 2018 did not match IRS records. I still have not received my refund and have received no record of whether IRS has received my return.

Sorry the IRS has limited staff due to Covid-19.  They just started opening the mail in June that's been stacking up. Back in March the IRS closed their processing centers and stopped processing mailed tax returns.  An IRS spokesperson recently said that they have about 11 million pieces of unopened mail.  So you have to keep waiting.

See this IRS page for the latest information

https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue