Pixel 5 and Pixel 4a 5G dual SIM support has been improved in March update
(Android Police)

You can now use 5G on both a physical SIM and eSIM at once Monday`s feature drop for the Pixel phones included a fun nugget that wasn`t in the official changelog: it appears you can now use both a physical SIM card and an eSIM on 5G networks at the same time. Previously users who had both installed and operational were limited to LTE speeds, even if their carrier had 5G service in the area. According to several commenters on the Pixel subreddit, 5G service has appeared for those users after receiving the March OTA and feature drop. Read More Pixel 5 and Pixel 4a 5G dual SIM support has been improved in March update was written by the awesome team at Android Police.