I would recommend that you take it back to where you purchased it so that it can be checked for any defects by a licensed Apple technician. Genius Bar Reservation and Apple Support Options - Apple
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You are lucky !!
My phone in closed areas (eg Inside a conference room) has no signal at all (No Service) Vs other iphone users (like 6,7) that have full signal (under same carrier, same spot - comparing apples to apples).
Therefore, it seems (based on feedback I get from my colleagues that also have iPhone X) that iPhone X has antenna issues ....same on you Apple !!!
Question: iPhone X cellular signal bars