Ok ... you have 2 separate issues.
Until they process the 2019 return the stimulus check will not be issued and there is nothing you can do about it but wait. If they do not issue the check by 12/31 then you will be able to apply for the credit when you file the 2020 return.
Next the increase in Medicare premiums due to the 2018 return AGI ... this is normal that the 2021 cost is based on the return 2 years back ... this is the norm. So the 2018 return you filed in 2019 is the baseline for the 2021 increase. The 2019 return has nothing to do with this. NOW ... if the 2018 income was unusually higher than normal for a one time situation like you sold a property, sold a stock, took out a large pension/IRA distribution or something like that then you can contest the increase ... follow the instructions in the notice you received. https://www.aarp.org/retirement/social-security/questions-answers/medicare-premium-irmaa.html
