iPhone 11 show only 2-3 signal bars when using LTE, caused me missed a call

So the problem appeared a month ago, right after I bought an iPhone 11 at an AAR in my place. I immediately noticed the signal showed only 2 bars. They told me it's just a normal situation, the signal would get better soon.

Until the last Sunday (June 27), I went downtown to hangout with my friends. They told me they tried to call me but failed (perhaps due to my phone had no signal back then). I also received a message from my mobile carrier about the failed call.

I started feeling nervous about the situation. I realized all the time since I began to use this phone, the signal was low. Here are some info that I noticed:

• It happens mostly with LTE. Sometimes when I receive a call, LTE turns into 3G and the signal goes strong.
• It's weird that I usually gets bad signal when being outside, especially when I missed a call from my friend, I was in a wide open space with no high buildings around. This seems opposite to normal cases.

I went to my mobile dealer to see if they can tell the problem. However they just told me to bring my phone in, they will send to apple center to check. Maximum wait time is 15 days. I don't like to wait that much. So I tried to test out some situations to find out the problem myself before I get in their store asking for help again.

Things I tried but failed to address this issue:

• Change my SIM card to other device (my old iPhone 6s) => My iPhone 6s also got bad signal. I guess it's about the SIM card and mobile carrier. So I tried:
• Get a new SIM card but same number => No work.
• Bought a new SIM card, from a different mobile carrier => No work. Same low signal.

The only thing I haven't tried yet is putting my SIM cards into other cellphone brands rather than Apple iPhones. Will try that.

Reason for this test started this morning, when I was working with my mobile carrier, a random guy told me that he experienced the same situation due to using iPhones. I show me both of his devices, one iPhone and one other brand I don;t know, the signal strength is full in the other brand, same low signal on an iPhone, just like me.

So anyone knows what's the problem and how to fix it? Any input is appreciated. Sorry for my bad English though.