We've all been there. Pedals can be difficult sometimes.

The R540 pedals have wrench flats on them, so I suggest that you use them, instead of the hex fitting inside the axle. Either approach is OK, but I find it easier to get leverage on an open-ended wrench than a hex tool (even if on a ratchet), mostly because you aren't working "behind" the crank arm where it can be tough to get positioned (and you don't have to think "opposite" for the direction to pull the wrench).

Ideally use a genuine pedal wrench, which is really just a 15mm open end wrench with two distinguishing features: a somewhat thinner profile (to fit into the narrow faces on the pedal), and a longer arm (for leverage). Lacking that, if you have a 15mm wrench thin enough to get onto the pedal faces, you can always add leverage with a pipe slid over the wrench.

Andrew's advice to put the chain on the large ring is good advice -- I know I've drawn blood when I've skipped that step and the wrench finally responds to force on the drive side!

Left side is left threaded as noted by fietsbob, so any effort may have made there assuming conventional threads only tightened it further.

Lastly, it can help to get the opposite side crank is a good position for opposing the wrench. Hence, the first pedal tends to come off easiest, since you can step on the opposite side pedal to counter your wrench. But once the first pedal is off, no more pedal to step on for opposing your second effort.