Hi - I am working with panel data (US industrials, 2000-2019) and am researching the impact of firm geographic diversification (GSD) on firm performance (ROA.) Fixed effects repression suggests an inverted U relationship between GSD and ROA. The linear and quadratic relationship of GSD performance are both significant. As a robustness check, I used 2SLS to correct for endogeneity of the regressor GSD (linear and quadratic.) The post estimation tests suggested that underidentification, weak identification annd over identification were not a problem and the regressor did in fact have an endogeneity issue. However, the results of 2SLS suggest non-significant relationship between GSD and performance. I am wondering how to interpret this result. In case you have any suggestions, please do let me know. Thank you.

Code:
. xtivreg2 Ln_EBIT_ROA Ln_Revenue Ln_LTD_to_Sales Ln_Intangible_Assets CoAge wGDPpc wCPI wDCF w
> Expgr wGDPgr wCons Ln_PS_RD (l1.Ln_GSD l1.Ln_GSD_Sqd= l1.Ln_Indgrp_GSD_by_Year Ln_Int_exp Ln_
> Int_exp_Sqd l1.Ln_ROS) if CoAge>=0 & NATION=="UNITED STATES" & NATIONCODE==840 & FSTS>=10 & G
> ENERALINDUSTRYCLASSIFICATION ==1 & Year_<2020 & Year_<YearInactive & Discr_GS_Rev!=1, fe endo
> g (l1.Ln_GSD)
Warning - singleton groups detected.  35 observation(s) not used.

FIXED EFFECTS ESTIMATION
------------------------
Number of groups =       141                    Obs per group: min =         2
                                                               avg =       5.7
                                                               max =        17

IV (2SLS) estimation
--------------------

Estimates efficient for homoskedasticity only
Statistics consistent for homoskedasticity only

                                                      Number of obs =      798
                                                      F( 13,   644) =     4.41
                                                      Prob > F      =   0.0000
Total (centered) SS     =  203.9428465                Centered R2   =  -0.0686
Total (uncentered) SS   =  203.9428465                Uncentered R2 =  -0.0686
Residual SS             =  217.9269693                Root MSE      =    .5759

--------------------------------------------------------------------------------------
         Ln_EBIT_ROA |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
---------------------+----------------------------------------------------------------
              Ln_GSD |
                 L1. |  -.7524867   .4739275    -1.59   0.112    -1.681367    .1763941
                     |
          Ln_GSD_Sqd |
                 L1. |    .108648   .1192194     0.91   0.362    -.1250177    .3423138
                     |
          Ln_Revenue |   .5130579   .1374392     3.73   0.000      .243682    .7824338
     Ln_LTD_to_Sales |  -.1517859   .0352142    -4.31   0.000    -.2208044   -.0827674
Ln_Intangible_Assets |  -.0811832     .04624    -1.76   0.079    -.1718119    .0094456
               CoAge |  -.0249093   .0143349    -1.74   0.082    -.0530053    .0031866
              wGDPpc |   .0000639   .0000309     2.07   0.039     3.33e-06    .0001244
                wCPI |  -.0016804   .0280792    -0.06   0.952    -.0567146    .0533538
                wDCF |  -2.80e-15   1.67e-13    -0.02   0.987    -3.29e-13    3.24e-13
              wExpgr |   .0132025   .0114394     1.15   0.248    -.0092182    .0356232
              wGDPgr |  -.0291117   .0338202    -0.86   0.389    -.0953981    .0371747
               wCons |   2.88e-14   6.23e-14     0.46   0.644    -9.32e-14    1.51e-13
            Ln_PS_RD |  -.0570622   .0723795    -0.79   0.430    -.1989234     .084799
--------------------------------------------------------------------------------------
Underidentification test (Anderson canon. corr. LM statistic):          45.779
                                                   Chi-sq(3) P-val =    0.0000
------------------------------------------------------------------------------
Weak identification test (Cragg-Donald Wald F statistic):               12.021
Stock-Yogo weak ID test critical values:  5% maximal IV relative bias    11.04
                                         10% maximal IV relative bias     7.56
                                         20% maximal IV relative bias     5.57
                                         30% maximal IV relative bias     4.73
                                         10% maximal IV size             16.87
                                         15% maximal IV size              9.93
                                         20% maximal IV size              7.54
                                         25% maximal IV size              6.28
Source: Stock-Yogo (2005).  Reproduced by permission.
------------------------------------------------------------------------------
Sargan statistic (overidentification test of all instruments):           2.186
                                                   Chi-sq(2) P-val =    0.3352
-endog- option:
Endogeneity test of endogenous regressors:                               8.598
                                                   Chi-sq(1) P-val =    0.0034
Regressors tested:    L.Ln_GSD
------------------------------------------------------------------------------
Instrumented:         L.Ln_GSD L.Ln_GSD_Sqd
Included instruments: Ln_Revenue Ln_LTD_to_Sales Ln_Intangible_Assets CoAge
                      wGDPpc wCPI wDCF wExpgr wGDPgr wCons Ln_PS_RD
Excluded instruments: L.Ln_Indgrp_GSD_by_Year Ln_Int_exp Ln_Int_exp_Sqd
                      L.Ln_ROS
------------------------------------------------------------------------------