As mentioned earlier, a dual frequency GNSS receiver is a nice piece of hardware, but the surrounding equipment must also match. So if using Tallysman TW2712 antenna you will only be able to receive L1 frequency:
Fortunately, I found a used Maxtenna M1227HCT-A2-SMA at low cost (45 €) on Ebay. As expected, the L2 frequency can be received by using this antenna:
By using my antenna splitter receiver setup, I started a half hour test run:
The DataGNSS receiver was able to get a RTK-Solution in about 24 seconds and was able to keep the position extremely precise over the whole run.
The U-Blox receiver got a RTK-Solution after 160 seconds, was less stable and drifted away.
Conclusion: When it comes to precision, quality of hardware matters.