Is it unethical to use/exploit a glitch on large corporation's (T-Mobile) service to get (part of) the service for free instead of notifying them?

(This didn't require me to workaround anything, it's just their bad — for them — setup.)

So, for around the past 8 months I've been using a cheap IoT SIM card from T-Mobile with some tiny high-speed data allowance and 64kbps unlimited.
The 64 kbps unlimited turned out to just stay high-speed unlimited, in 2 countries at least. Ironically, I had trouble even just activating (switching to — only sold as physical) the eSIM in the home country, so I couldn't test it there.
I've been using it mostly as a backup alongside another regularly paid plan at first, since it can connect via 3 MNOs in my country, but I also used it a bit more to save some money later after my main carrier increased the price by 30%. I've been trying to keep it up to around 20GB/month per SIM, since that is the theoretical limit with 24/7 64kbps, but this month I overshot it with 35GB.
Currently I switched to ad-supported low-speed eSIM instead since that's good enough for now.

I also have a second SIM in modem connected to my mini PC I wanted to play around with remotely, but I didn't really get to much.

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wwan0  /  monthly

        month        rx      |     tx      |    total    |   avg. rate
     ------------------------+-------------+-------------+---------------
       2025-03    766,87 MiB |    9,46 GiB |   10,21 GiB |   32,74 kbit/s
       2025-04    753,07 MiB |    8,95 GiB |    9,68 GiB |   32,08 kbit/s
       2025-05      9,87 MiB |    1,73 MiB |   11,60 MiB |       36 bit/s
       2025-06     42,38 MiB |   15,13 MiB |   57,52 MiB |      186 bit/s
       2025-07      1,21 GiB |   24,45 MiB |    1,23 GiB |    4,06 kbit/s
     ------------------------+-------------+-------------+---------------
  

  

€1.60/month. Cheap enough even if I don't use it.

Other than that, I also try to always stay connected to MNOs under Deutsche Telekom umbrella, since I expect they're gonna have the lowest roaming fees there. It even appears to be the default, but sometimes I need to help it if the signal is worse.
The current EU cap is €1.30/GB for what the roaming partner can charge.
And actually, they can charge me up to this value when roaming permanently (+VAT), but they haven't yet done so (it's prepaid, so there won't be extra bills later).

But also, this is a service I couldn't otherwise get locally. As a young person (up to 28), I can get 300GB/month from Telekom for just €20.50, but there's no domestic roaming for better coverage like I am getting here.
The only carrier which used to offer it was O2, but they discontinued it because it apparently worsened their reputation. Apparently, the competition was telling their customers that they have domestic roaming due to "having to rely on other's networks", from what I've heard.

I know of other similar past glitches. Embeddedworks IoT SIM using the T-Mobile network used to have actual unlimited speed instead of 64kbps, but this has since been fixed.
15GB T-Mobile roaming pass could somehow be overshot, I've seen a screenshot saying "24GB out of 15GB" used.
Firsty free eSIM could do unlimited speed to google services like maps and YouTube (tested with 4k60 video), but since that's a smaller company offering free service, I've reported it to them, and it has been fixed since switch from KPN to Proximus.

Others in certain discussions also mentioned similar glitches with a few other unknown providers, but since too many people using those would get it fixed quickly, the specifics are usually not mentioned.