If you are looking for compact antennas these may suit your needs, they are only about 5 inches long. I tested them and got good results in close proximity to the modem. I can't judge distances over 20 feet. Inexpensive.
📶 Proxicast Ultra Low Profile MIMO 3G / 4G / LTE Omni-Directional Screw-Mount Antenna for Verizon, AT&T, Sprint (and Others) Modems & Routers with SMA External Antenna Ports
9 Review
📡 MikroTik LHG 5: High-Gain Dual Polarization Grid Antenna, Enhanced 5 GHz Performance (US Version)
9 Review
Gadgets World FBA_ALFA ARSN19M OMNI Directional High Gain
9 Review
📡 Eightwood Dual Band WiFi Antenna with RP-SMA Male Connector, 2.4GHz and 5GHz Frequencies, Magnetic Base for PCI-E WiFi Network Card, USB WiFi Adapter, Wireless Router
9 Review
UGREEN USB 3.0 Ethernet Adapter Hub with RJ45: Fast Gigabit Ethernet Converter, 3 Ports USB 3.0 Hub Compatible for MacBook, iMac, Surface Pro, Chromebook, Laptop, PC
11 Review
🔌 StarTech.com NETRS232 Serial to IP Ethernet Device Server - DIN Rail Mountable - Serial Device Server - Serial Over IP Device Server (Black)
4 Review
🌐 StarTech.com NETRS2321P: 1-Port RS232 to Ethernet IP Converter, Serial over IP Device Server - Black
5 Review
HB-UMLS Sabrent USB 2.0 Hub with 4 Ports and LED Power Switches for Each Port
12 Review