@coffeewithbren I don't know what's the situation in Baler today, but back in 2017/8, Internet connection there is slow and intermittent. You have to go outside and find the nearest cell tower to get a decent connection (but still intermittent).
That's a few years back, so it might be far better today. (Oh, back then, Globe had better coverage than Smart. Not sure now.)
However, since the heat is generally coming from the Pacific Ocean, blowing through the Philippine Sea, then inland, it is probably hotter in Aurora province. Then again, you're planning an August trip, if things get better by then, Baler will be a good option.
La Union. Last I went there was in the 90s. The last time I passed by the area was in 2019. Too crowded with establishments and tourists. It is no longer the La Union that I loved. From a friend, Internet in the area is very good and stable. Actually, the entire Ilocos Region (Region I) has a very good Internet infrastructure. It just that, if you like to stay away from crowds, it will be hard to do so unless you enter residential areas and go outside the usual tourist spots, literally go local.
Zambales. Nope. No info to share. Last time I went there was in the mid 90s.
You might want to consider Bataan, especially if you want to reconnect with nature and history. Internet's good. Though the closer you get to the mountainous areas, the more challenging it will become. Trees + mountains are not a good combination for mobile signals. Maybe they improved the infrastructure, but, quiet environment, Bataan is a good place to consider.