v="Version 0.2.0 b183"
[[ "$v" == *"b183"* ]] && echo "OK" || echo "FAIL"
[[ "$v" =~ "b183" ]] && echo "OK" || echo "FAIL"

Both should display "OK", but the first show "FAIL" and the 2nd gives "ash: =~: unknown operand"

What can I do instead?

v="Version 0.2.0 b183"

[[ "$v" == *"b183"* ]] && echo "OK" || echo "FAIL"
# You compare the "Version 0.2.0 b183" string with the "*b183*" string - so they are not equal.

# One of the many options:
[[ `echo "$v" | grep -o b183` ]] && echo "OK" || echo "FAIL"


[[ "$v" =~ "b183" ]] && echo "OK" || echo "FAIL"
# `=~` ash has no such operand.

[[ "$v" == *"b183"* ]]

Using wildcards is working in standard Linux e.g. Ubuntu - but apparently not on the Onion.

But the "grep" line is working

Thanks!

Using wildcards is working in standard Linux e.g. Ubuntu - but apparently not on the Onion.

Yes.
Please install bash - if you can't live without it - and you can use it in bash script in the usual way.
(You know, the shebang is: #!/bin/bash).

For example:

# Omega2+ v0.3.2 b234

root@Omega-99A5:~# opkg update
 ...
root@Omega-99A5:~# opkg install bash
Installing bash (4.4.18-2) to root...
Downloading http://repo.onioniot.com/omega2/packages/packages/bash_4.4.18-2_mipsel_24kc.ipk
Configuring bash.

# -----------------------------------------------------------------------

root@Omega-99A5:~# cat test.sh
#!/bin/bash

v="Version 0.2.0 b183"

[[ "$v" == *"b183"* ]] && echo "Test_1: OK" || echo "Test_1: FAIL"
[[ "$v" =~ "b183" ]] && echo "Test_2: OK" || echo "Test_2: FAIL"

# -----------------------------------------------------------------------

root@Omega-99A5:~# ./test.sh
Test_1: OK
Test_2: OK