Please how can I fix my IPhone 8 service problem

See these Apple articles below:

If you see No Service, Searching, or SOS on your iPhone or iPad

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)

Quincyjez ~ 77% is not healthy and is to be expected in your old model of iPhone. Have the weak, aging battery replaced and all will (probably) be well:

iPhone Battery & Power Repair

About genuine iPhone batteries

Repair Provider Verification

(Articles found by searching Apple's Knowledge Base.)

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Quincyjez wrote: "Does battery has anything to do with phone network?..."

Yes. iPhone's antenna requires strong battery power to connect to a network. Apple suggests replacing the battery when its maximum capacity is ≤80%.

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Quincyjez wrote: "I even try updating the latest version but it didn't allow me install it saying i disconnect the network even when i did not"

It's not surprising with a weak battery capacity of 77%.

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Quincyjez ~ Does your iPhone 8 still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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Quincyjez wrote: "...can that be the same issue with the missing IMEI?"

I don't know — ask Apple when you go to have the battery replaced.

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