Dual SIM iPhone Cannot Send Texts without iMessage Enabled

I wish to send SMS's but without using iMessage.

I originally used a physical SIM card for personal calls, from my original provider, Optus.

I now have a second phone number for work, with SIM, plan and number provided by Telstra.

I only want to carry one phone for personal and work.

I changed my physical SIM for an electronic SIM with my Personal provider.

At this point, phone worked fine and would send SMS without iMessage.

I then installed a physical SIM for my Work provider.

After this, standard SMS's would not send. The screen simply failed to respond to the "Send" icon.

Once iMessage was enabled, SMS's would send okay but I want to send texts without iMessage.

What can I do? All other aspects of the dual SIM phone work fine.

My phone is an iPhone 11.

Can I please ask that any responder reads and understands the issue fully, before replying?

Many thanks in advance.