Testing Three 4G broadband - curious speed test results

I've been recently testing (and usefully using) a low cost, 4G backup to my occasionally problematic (fixed now, hopefully) FTTC. The equipment is:

- A Huawei B311 router.
- A Smarty SIM.

Usable, but I get weird speed test results:

- Slow (2-3Mbps) down.
- Crazy fast uploads (12-15Mbps).

Based on previous experience (BT/EE 4G and more recently a 5G phone), I would normally expect this to be the other way round. Any ideas? I've read Three is notorious for congestion.... A whole lot of downloading/torrents going on, but not a lot of upload?

I might try with an EE SIM card - more expensive as a backup goes (at least for the very minimum ongoing costs when the backup isn't in use) as I am at least familiar with their performance local to me.

A higher LTE category router might help. Before you buy test with a modern smartphone.