Honestly, if your diet is as good as you say you should be gaining weight. If you're not making gains you're simply not eating enough. With your range for test at 300 that's not terrible, but it's enough to build muscle lol.
I say this coming from the same situation. I had basement level test when i started lifting hard, and i thought that was my problem. It wasn't. The second i started downing 4k cals a day i gained weight. And i started at 6'1 155. I thought i was eating in surplus, i was counting cals, but i simply wasn't eating enough. Fast food and ranch and chocolate was the key to my initial gains. As much calorie dense food as i could find, and lots of it.