Phone numbers missing from SD card copy of contacts.

I am experiencing compounding problems with the actual phone numbers going missing or dropping out from contacts in my M11 Android 10 mobile phone.
Everything worked fine until I added data connection to my phone; I previously only had a phone call/text service.
I have updated my phone contacts to the SD card. This works, but the phone numbers (which are most important to me) are missing.
I have sync enabled and some of my contacts now show in my gmail account page, but not all of them!
(This is most strange??).
I have read a number of websites describing how to address fixing missing contacts, but none have worked so far.
Can someone assist me with this please?

EDIT:

Solved!

 

Thanks for the reply dvhttn.
I really meant SD card and I have read up on the limitations of saving contacts on both the SIM and SD card.
The fact is that I have reached a point of desperation as the "loss of phone numbers" problem is far more general that I have described so far.
So much so, that I have taken to writing numbers down in a phone book, old fashioned style.
For years I used a phone only SIM card service with non of these problems; they have only manifested since I "upgraded" to a data plus phone call SIM.
I am now seriously considering going back to the phone call/text message only service.
I've spent many hours reading instructional websites on how to fix this; to absolutely no avail.
Also, I have read several old threads describing the same, or similar problems, with no resolution.
That is less than encouraging.

If there is something completely fundamental that I am missing, I would very much like to know.

Jim.

 

Danny,
Thanks for the reply with detailed advice. I did have the Google Contact application installed.
While it may appear to be totally illogical, this is what I am trying:

1. I have started to save all known phone numbers in "Phone Contacts" only.
2. I have turned sync OFF.
3. If this does succeed, I will then need to find out how to backup my phone contacts without the use of Google Contacts.

So far, (only 24 hours), no numbers have disappeared.
I know all the advice I have read on multiple websites stress to turn ON sync, but I am suspecting that is where the glitch was originating from.

Time might prove me totally wrong. If I am totally confused and off track, (very possible), please feel free to tell me so!
More later.

Jim.

 

This solution that I have resorted to seems to be providing me with a stable list of Contact phone numbers, as I have described above.
Other users have taken this approach as described further on this thread:

https://androidforums.com/threads/how-to-separate-phones-and-gmails-contacts.1145575/

I will now edit this thread as solved.

Thanks for all replies.

Jim.