The RTL8157 chipset works with 2.18-1-1.
2.18-1.1 package shows the RTL8157 chipset in action.

I actually confirmed that data transfer at 5GbE speed is possible.
However, I can't check the link speed connected to 5000Mbps (5GbE) in Synology's ethtool or Synology network information, and I can't modify the MTU.

This problem is unlikely to be fixed right away, but I'll leave it here for reference.

The product used is WisdPi WT-UT5.

• RTL8152/RTL8153 driver [2.18-1-1]
• Synology DS920+ [DSM 7.2.2-72803]

|__2-2 0bda:8157:3000 00 3.20 5000MBit/s 544mA 1IF (WisdPi USB 5G Ethernet 000334C******10017)

eth2 Link encap:Ethernet HWaddr 34:::B1::17
inet addr:10.1.0.* Bcast:********* Mask:255.255.255.0
inet6 addr: fd3b::bee2::36c8:d6ff:feb1:17/64 Scope:Global
inet6 addr: fe80:::d6ff:*:17/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:141772521 errors:0 dropped:0 overruns:0 frame:0
TX packets:390506389 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:198520946899 (184.8 GiB) TX bytes:552978638476 (515.0 GiB)

Settings for eth2:
Supported ports: [ MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
2500baseX/Full
Supported pause frame use: No
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
2500baseX/Full
Advertised pause frame use: Symmetric Receive-only
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Link partner advertised link modes: 100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Link partner advertised pause frame use: No
Link partner advertised auto-negotiation: Yes
Link partner advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Full
Port: MII
PHYAD: 32
Transceiver: internal
Auto-negotiation: on
Cannot get wake-on-lan settings: Operation not permitted
Current message level: 0x00007fff (32767)
drv probe link timer ifdown ifup rx_err tx_err tx_queued intr tx_done rx_status pktdata hw wol
Link detected: yes