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
Gadgets World FBA_ALFA ARSN19M OMNI Directional High Gain
9 Review
Asus RP-N12 N300 WLAN Repeater 🔁 802.11 B/G/N - Boost Your Wireless Range!
10 Review
4G LTE modem MikroTik LHG LTE6 kit white
38 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
🔌 USR-TCP232-410s: RS232/RS485 Serial to Ethernet Adapter/IP Device Server with DHCP/DNS Support
4 Review
HB-UMLS Sabrent USB 2.0 Hub with 4 Ports and LED Power Switches for Each Port
12 Review
🔌 uni USB C Hub with Ethernet Adapter, 4K HDMI, Gigabit Ethernet, and 3 USB 3.0 Ports for MacBook Pro, iPad Pro, XPS
11 Review