Signal message delivery delayed on LTE/5G with 1&1 (Germany)

[ltstereo]

Guidelines

• I have searched searched open and closed issues for duplicates
• I am submitting a bug report for existing functionality that does not work as intended
• This isn't a feature request or a discussion topic

Bug description

I have noticed that Signal messages are reliably delivered only when connected to a Wi-Fi network. Once I switch to LTE or 5G, I experience the following issues:

Messages are significantly delayed.
This problem occurs exclusively when using the mobile carrier 1&1 (Germany).
Other messaging apps like WhatsApp and Telegram work flawlessly under the same conditions.

Steps Already Tested:

Checked energy settings: I ensured that Signal is allowed to run unrestricted in the background. No improvement.
Tested with a different device: The same issue occurs when using a different phone with a 1&1 SIM card.
Tried multiple SIM cards:
    1&1: Messages are delayed or not delivered.
    Telekom/Vodafone: No issues – messages are delivered immediately.
Reinstalled and reconfigured Signal: The issue persists.
Tested in different regions: The problem occurs regardless of the location or network coverage.

My Hypothesis:

The issue seems to be tied specifically to the mobile carrier 1&1, as:

Signal messages are delayed even on other devices using 1&1.
Other providers like Telekom and Vodafone have no such problems.
Other messaging apps like WhatsApp and Telegram function as expected.

Could it be possible that Signal traffic is being delayed or blocked by 1&1? Perhaps specific Signal protocols are being affected?

Expected Behavior:

Messages should be delivered promptly, just as they are when using other mobile carriers or when connected to Wi-Fi.
Actual Behavior:

Messages are delayed or not received at all when using LTE/5G with 1&1, but work fine with Wi-Fi or other mobile carriers.

Screenshots

Device

Pixel 4a

Android version

Andriod 13, LineageOS 21

Signal version

7.25.2

Link to debug log

From the sending phone:
https://debuglogs.org/android/7.24.2/4c39225df95c748d48ccabbfc32b1f20444011e2f6804b2180cd5b5ddb119bae

From the receiving phone (1&1):
https://debuglogs.org/android/7.25.2/61a1dd33dcac274f46222221117182babfd194e52e68d3812c2c6edd157b9df9