How can i check my messages from another phone?

You do not need monitoring software for this: sign your own account in on the second device and the message history syncs to it. That works because the messages belong to your number or your account, not to the handset.

Free ways to reach your own messages from a second device.

• Google Messages for web (messages.google.com/web) — scan the QR code from the phone's Messages app, and your SMS and RCS threads open in any browser. Pairing lasts until you unlink it.
• WhatsApp Web / desktop — Linked Devices from the phone's settings, up to four companion sessions.
• iMessage on a Mac or iPad signed into the same Apple Account, if Text Message Forwarding is enabled on the iPhone.
• Carrier web portals — Verizon Messages+, T-Mobile DIGITS and similar mirror SMS to a browser on request, free on most plans.
• Google backup — a Google One backup of the phone includes SMS, restorable to a new device.

The distinction that matters: reaching your own messages on your own second device is ordinary account access. Reaching someone else's messages is not, and no app changes that. We covered the other side of that line in this thread.

If what you actually want is a family device. Ultimate Phone Spy covers Android 4.0+, Windows and Mac — the SMS module at https://ultimatephonespy.com/sms-tracker mirrors threads to a web dashboard, and child-safety scoping is set out at https://ultimatephonespy.com/child-safety-monitoring. It does not cover iPhone, and it requires a physical install plus the device owner's knowledge. Pricing is $69.90/month, $40/month quarterly or $16/month annual.

One caution. Google Messages pairing and WhatsApp linked devices are the two things attackers use most to read someone's texts remotely — if you see an unfamiliar session in either list, unlink it and change the account password, then check https://myaccount.google.com/security for active devices.