As far as the Tel-Tru check the calibration. Boiling water with 2 kabobs and Thermapen tip in the water should read 100℃. If not recalibrate with the adjustment nut.
But, still go off of IT of food.

First up-welcome aboard and enjoy the journey despite the rocky start.

Welcome to the club. We will all try our best to help you. The first time I cooked steak on my egg I put the meat on almost immediately after I lit the egg. Needless to say it tasted like lump. 

After one bite I knew I had screwed up. With patience I got there, and you will too. 

You really should cook to temp. Get a Thermapen.

When you put your meat on the temp will drop thats  why you have to go by internal temp not time

A $5 meat thermometer works just fine (and has for decades)

Gogogordy said:

A $5 meat thermometer works just fine (and has for decades)

no offence, just wondering if a Thermapen could have helped avert this disaster? 

Welcome @jcjayco‌, don't get discouraged, eggin' gets better and better with time. Try to enjoy the journey. +1 with what @johnkitchens‌ said about helping. Sometimes we just need a little more information to help out. Please let us know what types of cooks you are doing. i.e. Whole chicken, chicken parts, steaks etc.

I see that you're using a digital thermometer and said it appears to be accurate. A couple of things could be happening. 1. The thermo might not be accurate, especially if batteries are low. 2. Depending on the type of thermometer, you might not be letting it sit in the meat long enough to get an accurate temp. For instance, my ThermoPop is fast, not super Thermapen fast, but is rated at 5 sec. Fast enough that when I go to insert it into the meat, I get an instant reading from the surface temp of the meat. I need to let it settle to make sure I'm getting an Iternal meat temp and not a surface meat temp.

How much cooking experience do you have(outdoor/indoor)? This is in no way meant to be an insulting question. The reason I ask is because if you are a baker, focus on baking/smoking something indirect in the egg that you're comfortable with and can tell doneness without the instruments (like you do in the oven). Likewise, if your a stovetop or grilling person, focus on those things until you get comfortable with the egg. Your egg will last years, so you have plenty of time to expand your menu. Focus on the fun first.

I think this is one of the most important things. An internal temperature(IT) keeps you safe. A lot of people like things cooked to the minimal safe zone and will argue that's what taste best. Many people on this forum flirt with the danger zone to enjoy food to their liking. Can you explain your definition of undercooked? Take for example a pork chop that has a safe temp of 145°. My wife will not eat this because it has a pink hue and the texture's not right. She's used to eating the dried out chop that everyone grew up on because the fear of pork. The point I'm trying to make is, cook to your liking and use the thermometer to make sure your food is safe(unsafe food illness sucks). If you like the taste and texture of pork at 165°, cook it a little longer.

Again, welcome to the forum and remember to enjoy the journey.

NPHuskerFL said:

show previous quotes

A $5 meat thermometer works just fine (and has for decades)

So long as it's accurate. Yes it does.

Just because a device is high-tech, and electronic doesn't intrinsically make it more accurate....in fact by virtue of the extra complexity there are MORE ways it can be fail or be wrong. My point is, great chefs have prepared great food for far longer trusting a traditional "old school" meat thermometer than have ThermaPens, or other over-complicated temperature readers. I'm just saying often the difference between success and failure is the simplest and a $5 dollar temp reader works just fine....this isnt laboratory research it's BBQ.

Are you referring to undercooked by your preference or undercooked by internal temperature? If undercooked to your preference, you will have to up the internal temp until you find a place you like. This is a subjective point. Most of the members here tend to like beef cooked to the lower edge of the "safe" scales (steaks) but take certain cuts that are tougher (brisket and pork butt) to the higher end to  render the fat and break down connective tissue to obtain a a tender product. Only you and your family can determine your personal preferences. Keep trying and you will find a happy medium for your taste.