Is there any other way to identify RICs for Fix Income instruments via TRTH v2 API?

You can use "Historical Criteria Search" to find Fixed Income RICs

For example; you can Search by maturity Date range.

Request:

POST https://hosted.datascopeapi.reuters.com/RestApi/v1/Search/HistoricalCriteriaSearch HTTP/1.1
Authorization: Token <your_auth_token_goes_here>
Prefer: respond-async
Content-Type: application/json; odata=minimalmetadata
{
    "Request": {
        "BondTypeCodes": null,
        "ContributorIds": null,
        "CountryCodes": null,
        "CurrencyCodes": null,
        "ExchangeCodes": null,
        "FutureMonthCodes": null,
        "InstrumentTypeCodes": null,
        "OptionMonthCodes": null,
        "OptionTypeCodes": null,
        "CouponRate": {
            "Min": 30
        },
        "StrikePrice": null,
        "ExpiryDate": null,
        "MaturityDate": {
            "Start": "2016-09-01T00:00:00.000Z",
            "End": "2016-09-30T23:59:59.000Z"
        },
        "Range": {
            "Start": "2015-09-30T00:00:00.000Z",
            "End": "2016-09-30T00:00:00.000Z"
        }
    }
}

Response

HTTP/1.1 200 OK
{
    "@odata.context": "https://hosted.datascopeapi.reuters.com/RestApi/v1/$metadata#Collection(ThomsonReuters.Dss.Api.Search.HistoricalSearchResult)",
    "value": [
        {
            "Identifier": "984308CM6=RRPS",
            "IdentifierType": "Ric",
            "Source": "",
            "Key": "VjF8MHgzMDAwMDAwMDAwMDAwMDAwfDB4MzAwMDAwMDAwMDAwMDAwMHx8fHx8fHw5ODQzMDhDTTY9UlJQU3w",
            "Description": "Historical Instrument",
            "InstrumentType": "Unknown",
            "Status": "Valid",
            "DomainCode": "6",
            "FirstDate": "2015-11-26T00:00:00.000Z",
            "LastDate": "2016-09-28T00:00:00.000Z",
            "History": [

            ]
        },
        {
            "Identifier": "AAHA2YEUAM=MP",
            "IdentifierType": "Ric",
            "Source": "",
            "Key": "VjF8MHgzMDAwMDAwMDAwMDAwMDAwfDB4MzAwMDAwMDAwMDAwMDAwMHx8fHx8fHxBQUhBMllFVUFNPU1QfA",
            "Description": "Historical Instrument",
            "InstrumentType": "Unknown",
            "Status": "Valid",
            "DomainCode": "6",
            "FirstDate": "2007-08-25T00:00:00.000Z",
            "LastDate": "2016-11-23T00:00:00.000Z",
            "History": [

            ]
        },
        {
            "Identifier": "AAHR1YGBAL=MP",
            "IdentifierType": "Ric",
            "Source": "",
            "Key": "VjF8MHgzMDAwMDAwMDAwMDAwMDAwfDB4MzAwMDAwMDAwMDAwMDAwMHx8fHx8fHxBQUhSMVlHQkFMPU1QfA",
            "Description": "Historical Instrument",
            "InstrumentType": "Unknown",
            "Status": "Valid",
            "DomainCode": "6",
            "FirstDate": "2014-09-20T00:00:00.000Z",
            "LastDate": "2016-11-23T00:00:00.000Z",
            "History": [

            ]
        },
        {
            "Identifier": "AAHR1YUSAL=MP",
            "IdentifierType": "Ric",
            "Source": "",
            "Key": "VjF8MHgzMDAwMDAwMDAwMDAwMDAwfDB4MzAwMDAwMDAwMDAwMDAwMHx8fHx8fHxBQUhSMVlVU0FMPU1QfA",
            "Description": "Historical Instrument",
            "InstrumentType": "Unknown",
            "Status": "Valid",
            "DomainCode": "6",
            "FirstDate": "2014-09-20T00:00:00.000Z",
            "LastDate": "2016-11-23T00:00:00.000Z",
            "History": [

            ]
        }
        •••
    ]
}

Please see this tutorial for more info.
https://developers.thomsonreuters.com/thomson-reuters-tick-history-trth/thomson-reuters-tick-history-trth-rest-api/learning?content=13755&type=learning_material_item